$_POST is an array of any variables received via HTTP_POST. $_GET is an array of ... well, yeah. And there's also $_COOKIE. $_REQUEST is all of the above. All of these are there automatically, in global scope, and can't be modified.
The only reason (I can think of) to copy those variables out to something else is if you want to modify them. But that's not what's happening. He's just renaming them.
The amount of copy/paste is also impressive. I'm thinking total LOC could drop by ~30% just by refactoring without changing any functionality.