Quick A: Because you want to not change the argument, or you want to forward it, respectively.

c++ rvalue reference and const qualifier

Among the many benefits of const qualification is to make an API more understandable, example:

template<typename T> int function1(T const& in);
// clearly, the input won’t change through function1

With the introduction of rvalue references, one can benefit from perfect forwarding but often const qualifiers are removed, example:

template<typename T> int function2(T&& in);
// can explicitly forward the input if it's an rvalue

Apart from documentation, is there a good way to describe that function2 won’t change its input?