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

SO screwed up my answer

You don't even need to return a reference:

struct C { };

C f() {
  return C();
}

int main() {
  C a;
  f() = a;  // compiles fine
}

Because this behavior is quite surprising, you should normally return a const value or a const reference unless the user has a sensible intent to modify the result.