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

The commonest case is to implement things like operator[].

struct A {
    int data[10];
    int & operator[]( int i ) {
         return data[i];
    }
};

Another is to return a big object from a class via an accesor function:

struct b {
    SomeBigThing big;
    const SomeBigThing & MyBig() const {
         return big;
    }
};

in order to avoid the copying overhead.