Why do we need bidirectional synchronized methods?

As stated in the topic. Why do we need bidirectional synchronized methods? What real world use case does it solve? What happens if I don't use them?

In Hibernate's User Guide:
> Whenever a bidirectional association is formed, the application developer must make sure both sides are in-sync at all times.
The addPhone() and removePhone() are utility methods that synchronize both ends whenever a child element is added or removed.

Source - Hibernate User Guide

In one of Vlad's blog posts:
>However, we still need to have both sides in sync as otherwise, we break the Domain Model relationship consistency, and the entity state transitions are not guaranteed to work unless both sides are properly synchronized.

Source - Vlad Mihalcea Blog

Lastly, in Vlad's book - High Performance Java Persistance, page 216:

>For a bidirectional @ManyToMany association, the helper methods must be added to the entity that is more likely to interact with. In our case, the root entity is the Post, so the helper methods are added to the Post entity

However, if I use simple generated setters, Hibernate seems to work just fine as well. Furthermore, synchronized methods might lead to performance degredation.

Synchronized methods:

    public void joinProject(ProjectEntity project) {
        project.getEmployees().add(this);
        this.projects.add(project);
    }

Generates this:

Hibernate:
    select
        employeeen0_.id as id1_0_0_,
        projectent2_.id as id1_2_1_,
        teamentity3_.id as id1_3_2_,
        employeeen0_.first_name as first_na2_0_0_,
        employeeen0_.job_title as job_titl3_0_0_,
        employeeen0_.last_name as last_nam4_0_0_,
        employeeen0_.team_id as team_id5_0_0_,
        projectent2_.budget as budget2_2_1_,
        projectent2_.name as name3_2_1_,
        projects1_.employee_id as employee1_1_0__,
        projects1_.project_id as project_2_1_0__,
        teamentity3_.name as name2_3_2_
    from
        employees.employee employeeen0_
    inner join
        employees.employee_project projects1_
            on employeeen0_.id=projects1_.employee_id
    inner join
        employees.project projectent2_
            on projects1_.project_id=projectent2_.id
    inner join
        employees.team teamentity3_
            on employeeen0_.team_id=teamentity3_.id
    where
        employeeen0_.id=?
Hibernate:
    select
        projectent0_.id as id1_2_,
        projectent0_.budget as budget2_2_,
        projectent0_.name as name3_2_
    from
        employees.project projectent0_
    where
        projectent0_.id=?
Hibernate:
    select
        employees0_.project_id as project_2_1_0_,
        employees0_.employee_id as employee1_1_0_,
        employeeen1_.id as id1_0_1_,
        employeeen1_.first_name as first_na2_0_1_,
        employeeen1_.job_title as job_titl3_0_1_,
        employeeen1_.last_name as last_nam4_0_1_,
        employeeen1_.team_id as team_id5_0_1_
    from
        employees.employee_project employees0_
    inner join
        employees.employee employeeen1_
            on employees0_.employee_id=employeeen1_.id
    where
        employees0_.project_id=?
Hibernate:
    insert
    into
        employees.employee_project
        (employee_id, project_id)
    values
        (?, ?)

Notice additional select for Employee right after Projects were fetched. If I use simply employeeEntity.getProjects().add(projectEntity);, it generates:

Hibernate:
    select
        employeeen0_.id as id1_0_0_,
        projectent2_.id as id1_2_1_,
        teamentity3_.id as id1_3_2_,
        employeeen0_.first_name as first_na2_0_0_,
        employeeen0_.job_title as job_titl3_0_0_,
        employeeen0_.last_name as last_nam4_0_0_,
        employeeen0_.team_id as team_id5_0_0_,
        projectent2_.budget as budget2_2_1_,
        projectent2_.name as name3_2_1_,
        projects1_.employee_id as employee1_1_0__,
        projects1_.project_id as project_2_1_0__,
        teamentity3_.name as name2_3_2_
    from
        employees.employee employeeen0_
    inner join
        employees.employee_project projects1_
            on employeeen0_.id=projects1_.employee_id
    inner join
        employees.project projectent2_
            on projects1_.project_id=projectent2_.id
    inner join
        employees.team teamentity3_
            on employeeen0_.team_id=teamentity3_.id
    where
        employeeen0_.id=?
Hibernate:
    select
        projectent0_.id as id1_2_,
        projectent0_.budget as budget2_2_,
        projectent0_.name as name3_2_
    from
        employees.project projectent0_
    where
        projectent0_.id=?
Hibernate:
    insert
    into
        employees.employee_project
        (employee_id, project_id)
    values
        (?, ?)

No more fetching of employee.

Full code.

Controller.

@RestController
@RequestMapping(path = "${application.endpoints.projects}", produces = MediaType.APPLICATION_JSON_VALUE)
@Validated
public class ProjectsEndPoint {


    @PostMapping("add-employee")
    @ApiOperation("Add employee to project")
    public void addEmployeeToProject(@RequestBody @Valid EmployeeProjectRequest request) {
        LOGGER.info("Add employee to project. Request: {}", request);

        this.projectsService.addEmployeeToProject(request);
    }
}

EmployeeProjectRequest.

@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
public record EmployeeProjectRequest(
        @NotNull @Min(0) Long employeeId,
        @NotNull @Min(0) Long projectId) {
}

ProjectService.

@Service
public class ProjectsService {

    private final ProjectRepo projectRepo;
    private final EmployeeRepo repo;

    public ProjectsService(ProjectRepo projectRepo, EmployeeRepo repo) {
        this.projectRepo = projectRepo;
        this.repo = repo;
    }

    @Transactional
    public void addEmployeeToProject(EmployeeProjectRequest request) {
        var employeeEntity = this.repo.getEmployee(request.employeeId())
                .orElseThrow(() -> new NotFoundException("Employee with id: %d does not exist".formatted(request.employeeId())));

        var projectEntity = this.projectRepo.getProject(request.projectId())
                .orElseThrow(() -> new NotFoundException("Project with id: %d does not exists".formatted(request.projectId())));

        //This line can be changed with employeeEntity.joinProject(projectEntity);
        employeeEntity.getProjects().add(projectEntity);
    }
}

ProjectRepo.

@Repository
public class ProjectRepo {

    private final EntityManager em;

    public ProjectRepo(EntityManager em) {
        this.em = em;
    }

    public Optional<ProjectEntity> getProject(Long id) {
        var result = this.em.createQuery("SELECT p FROM ProjectEntity p where p.id = :id", ProjectEntity.class)
                .setParameter("id", id)
                .getResultList();

        return RepoUtils.fromResultListToOptional(result);
    }
}

EmployeeRepo.

@Repository
public class EmployeeRepo {

    private final EntityManager em;

    public EmployeeRepo(EntityManager em) {
        this.em = em;
    }

    public Optional<EmployeeEntity> getEmployee(Long id) {
        var employees = this.em.createQuery("""
                SELECT e FROM EmployeeEntity e
                JOIN FETCH e.projects p
                JOIN FETCH e.team t
                WHERE e.id = :id""", EmployeeEntity.class)
                .setParameter("id", id)
                .getResultList();

        return Optional.ofNullable(employees.isEmpty() ? null : employees.get(0));
    }
}

EmployeeEntity.

@Entity
@Table(name = "employee", schema = "employees")
public class EmployeeEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String firstName;
    private String lastName;

    @Enumerated(EnumType.STRING)
    private JobTitle jobTitle;

    @ManyToOne(fetch = FetchType.LAZY)
    private TeamEntity team;

    @ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.PERSIST)
    @JoinTable(schema = "employees", name = "employee_project",
            joinColumns = @JoinColumn(name = "employee_id", referencedColumnName = "id"),
            inverseJoinColumns = @JoinColumn(name = "project_id", referencedColumnName = "id"))
    private Set<ProjectEntity> projects = new HashSet<>();

    public EmployeeEntity() {
    }

    public void joinProject(ProjectEntity project) {
        project.getEmployees().add(this);
        this.projects.add(project);
    }

    public void leaveProject(ProjectEntity project) {
        project.getEmployees().remove(this);
        this.projects.remove(project);
    }

        ... Getters and Setters ...
}

ProjectEntity.

Entity
@Table(name = "project", schema = "employees")
public class ProjectEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;
    private BigDecimal budget;

    @ManyToMany(fetch = FetchType.LAZY, mappedBy = "projects")
    private Set<EmployeeEntity> employees = new HashSet<>();

    public ProjectEntity() {
    }

    ... Getters and Setters ...
}

If there are really many elements on the Many side, then you probably should not use OneToMany at all. Fetching large collections implies using some kind of pagination\filtering, but OneToMany loads the whole set.

First of all, you need to update an owning entity(where FK resides) to store it in the DB. And what Vlad and Hibernate guide mean about consistency, refers to updating entity objects inside current session. Those objects have transitions during lifecycle, and when you have bidirectional association, if you don't set inverse side, then that inverse side entity won't have the field updated, and would be inconsistent with an owning side entity(and probably with the DB ultimately, after TX commits) in the current session.
Let me illustrate on OneToMany example.
If we get 2 managed entities Company and Employee:

set employee.company = X -> persist(employee) -> managed List<Employee> company.employees gets inconsistent with db

And there might be different types of inconsistencies, like getting from company.employees field after and arising side-effects(guess it was not empty, but just without employee you just added), and if there is Cascade.ALL, you might miss or falsely remove\update\add entities through broken relationships, because your entities are in a ambigious state, and hibernate deals with it in a defensive but sometimes unpredictable way:
https://stackoverflow.com/questions/22688402/delete-not-working-with-jparepository

Also, you might find interesting this answer: https://stackoverflow.com/a/5361587/2924122