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

In case you have a class that contains another structure, it can be useful to directly modify the contained structure:

struct S
{
    int value;
};

class C
{
    public:

        S& ref() { return m_s; }

    private:

        S m_s;
};

Allows you to write something like:

void foo()
{
    C c;

    // Now you can do that:

    c.ref().value = 1;
}

Note: in this example it might be more straightforward to directly make m_s public rather than returning a reference.