Bidirectional relationship between JPA entities in a hierarchy

I am using Spring Data JPA (with hibernate as the provider) for my project and wondering when do I need a bidirectional mapping between entities?

In my use case, I have a CategoryGroup entity having a @ManyToOne relationship to a Department entity. Currently, I don't have a corresponding @OneToMany relation in the Department entity. From what I could understand looking at other posts unless I need to delete via my entity/hibernate, I probably don't need to do it. I am wondering what else do I get by having a bidirectional relationship?

This is my Department excluding the accessors:

@Entity
  @Table(name="dbo.Department")
  public class Department 
  {
    	@Id
       	@Column(name="id")
       	private long id;
        	
       	@Column(name="name")
       	private String name;
  }

This is CategoryGroup excluding accessors:

@Entity
@Table(name="dbo.CategoryGroup")
public class CategoryGroup
{
	@Id
	@Column(name="id")
	private long id;
	
	@ManyToOne(fetch=FetchType.LAZY)
	@JoinColumn(name="dept_id")
	private Department department;
	
	@Column(name="name")
	private String name;
	
	public CategoryGroup()
	{  		
	}
	
	public CategoryGroup(String name, Department department)
	{
		this.name = name;
		this.department = department;
	}
}

As a follow up, my structure is hierarchical in nature. Category entity has a @ManyToOne relationship to a CategoryGroup entity and further on SubCategory entity has a @ManyToOne relationship to a Category entity.

If I keep adding bidirectional relation mapping on the parent side of the relationship then would that mean that once I retrieve the Department I will end up getting the whole entity hierarchy right upto SubCategory? For my use case that won't be desirable. Will marking the relationship with FetchType.LAZY alleviate that?

Thanks

OneToMany and ManyToMany associations are lazy by default, so you will not fetch the whole hierarchy when you add these associations.

Generally, adding these associations can greatly help you with querying as it reduces the need for explicit entity joins based on the join condition. In addition to that, you can define delete cascading if you need that i.e. if a Department is deleted you might want to delete all CategoryGroups associated with it.

Apart from all that, it might be desireable to explicitly separate domains from a modelling point of view. You might want a Deparment to be isolated i.e. don't know anything about the objects that are connected to it.