Well, anywhere BETWEEN the hashes is easy - teams trying a scrimmage play often want a wide side of the field.
As for anywhere BEHIND the three yard line - I can only guess it's for kickers. A kick from slightly farther back is actually easier from one that is too close, because of the lift angle needed on the kick. But in reality, the difference isn't THAT great, it's just a mental thing.
But when was the last time you saw a team request the ball farther back than the three yard line?