Why to make an interface in Presenter if we can just return a value and set in a View (MVP Structure)

It has been two months, i have been working with MVP Structure for creating Android apps.

As explained in every link i found regarding MVP that Presenter class takes care of all business login and data(from Model class), I understood that and started working.

One of the advantages shown in tutorials, i want to emphasise it here that MVP make unit testing easier because there is no dependency of views(Fine i understood that also).

What i did not understand is why to make interface to update views from presenter whereas i can just call a method of presenter that will return a value and i can set it there?

Lets come to the advantage i talked about above(Unit testing). Using those interface unit testing will be more problematic as method will require an interface implementation to complete an operation that we do not have in Unit testing(I know Instrumental test also comes in unit test but i m talking about only non-ui testing).

I just prefer to call a presenter method and get value and set in view in fragment or activity itself where as creation of interface creates another level of complexity and unnecessary interface declaration there all view operations are to be implemented. Its kind of frustrating.

One of the friend i am working with pointed out this problem in my code and told me to look all online references to clarify my mistake. But i want to know how these interfaces is help full in programming practice. Because i can not just digest it. Its becoming a pain in my ass. I have looked all the online references no solution.

Example

Presenter with interface

class Presenter
{

    private ViewInterface viewInterface;

    public void setViewInterface(ViewInterface viewInterface)
    {
        this.viewInterface = viewInterface;
    }

    // Here value is being passed to interface method that is implemented in fragment.
    // No problem with this implementation but why to do it. 
    // It will make unit test problematic as this method needs ViewInterface.
    public void calculate()
    {
        // Some calculation
        viewInterface.updateView(/*pass some paramerer*/);
    }
}

Presenter without interface

class Presenter
{


    // Here just take the value and set the view in fragment
    // Unit test easier just check the returned value.
    public int calculate()
    {
        int result = -1;
        // Some calculation
        return result;
    }
}

Fragment where presenter is being used

class MyFragment extends Fragment{

    ....

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.layout1, container, false);
    }

    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState)
    {
        super.onViewCreated(view, savedInstanceState);
        // just call method and get value to set in a view.
        int result = new Presenter().calculate();
        // set result in a view
    }

}

Please check comments in above code.

Any help will be appreciated. Thanks in advance.