Whats the significance of return by reference?

Question

In C++,

function() = 10;

works if the function returns a variable by reference.

What are the use cases of it?

Answer 1

It can be usefull when implementing accessors

class Matrix
{
   public:
      //I skip constructor, destructor etc

      int & operator ()(int row, int col)
      {
         return m_arr[row + col * size];
      }

   private:
      int size;
      int * m_arr;
}

Matrix m(10);
m(1,0) = 10;  //assign a value to row 1, col 0