This is, no doubt, a very very stupid question. However, bear in mind that to date I've never engaged in a battle with a real person, just the various CPUs.
So, bearing that in mind: What advantages does Substitute provide? Obviously it has them, or people wouldn't bother with it, but I can't see the point of the move, not from reading its description nor from watching other people's battles.
Sorry for the really dumb question.