Thanks for the google Prime_Solo
Achievement points can only be used for comparing yourself with other gamers. Usually a retail game has 1000 achievement points which can be unlocked. DLC can then add to that - as an ex. Mass Effect has 1000 point for being a retail game and 2 DLCs add 200 to that bringing it to a 1200 total.
Having maxed a game's achievements tell other gamers that you have played and completed more or less all that the game has to offer.
BTW: Do not mistake Achievement point for Microsoft points which are used for buying DLC online.
