Why does for_each return function by move

I was reading the documentation for std::for_each here http://ift.tt/1avWIYr and saw that the return value is std::move(f)

Why does the standard enforce moving the input parameter in the return value? Won't it be moved by default anyway, since the input parameter is passed by value?

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This leads me to a followup, when you compile the following code

Something function(Something something) {
    return something;
} 

The return statement is a move on my system with the highest optimization level (-O3), why don't most compilers elide this return value? Does C++17 enforce elision in this case? I read the proposal (http://ift.tt/1JFBM18) but I do not fully understand which cases qualify for mandatory elision.

I have tried this on Apple LLVM version 8.0.0 (clang-800.0.42.1) on my Mac and on g++ 5.4 on Ubuntu 16.04.