Yeah for example, There are some games that has extra game mods when you finish it (Dead rising 2, god of war etc.)
But what about getting all the achievements and they could give you something like an extra mission the fans would want (kinda like the dlcs on mass effect 2) or even a weapon for multiplayer (e.g. battlefield 2 had weapons when you bought the collector's edition)

Lol we seem to be straying off topic

I have only fully achieved mass effect 2 lol cause there weren't any impossible ones for multiplayer