Void type for method in Java OOP

I've had a problem for a few days now, I can't figure out what the Void method type is. What is the purpose of a method that does not return a value (what does it mean that the method does not have a return value of the method, my experience of that explanation is that the method does nothing)? I need it for the programming exam that is coming soon, and in the books it is always written the same way "Void method does not return a value".

I thought that methods with this type of return value only serve to redefine themselves. Maybe I have a problem because I associate the term "Does not return a value" with Mathlab, because in Mathlab if I put a colon at the end of the command, the command will be executed but will not return anything.

Matlab e.g.:

x = [ 90 0 0 ];
s=sin(x);

In simple terms, when we say that a method does not return anything, this implies that the method does not yield a value that can be assigned to a variable or be used in expressions.
The following is an example of such:

public class SumOfNumbers {
    public static void main(String[] args) {
        //you can choose to first initialize the numbers
        int num1 = 10;
        int num2 = 15;
        //call the method to add the two numbers
        add(num1,num2);

        //alternatively call the method and pass the numbers direct
        System.out.println();//to print new line between the two solutions
        add(10, 15);
    }

    //the method takes two numbers, finds the sum, prints the sum
    public static void add(int a, int b){
        int sum = a + b;

        //print the sum
        System.out.println("The sum of " + a +  " and "  + b + " is " + sum);
    }
}

note that in the main method, there is need for the output system as the method already has the output statement. the output system used in this code is to differentiate the two outputs i.e where we first initialise the numbers and when we pass the number direct to the method.

void method doesn't mean that it doesn't do anything, it means that its work is done without having to return a result.

This method performs a task but it does not give anything back.

for example, setter methods are good examples.
in this example, setFirstname is a method that assigns a value to the first name filed.

public class Person {
    private String firstname;

    public void setFirstname(String s) {
        firstname = s;
    }

    public String getFirstname() {
        return firstname;
    }
}

Let's use it

public class Main {
    public static void main(String[] args) {
        Person person = new Person();
        person.setFirstname("Ali");
        String personFirstname = person.getFirstname();
        System.out.println(personFirstname);
    }
}

the output will be:

> Ali

As you can see, when we use the getFirstname method and print its return value, it is clear that the firstname field has changed.

Who performs this task?
setFirstname method.

Although this method did not have a specific output, it did something important.