Sometimes there are various bonuses given in exchange for preordering. For example I saw a commercial yesterday for a special silenced shotgun if you preorder Splinter Cell from Gamestop, so these bonuses are sometimes actual in-game bonuses that are only available to people who preordered. Also, the bonuses often differ depending on which store you preorder from, so deciding the best place to preorder often comes down to shopping for which bonus appeals to you the most.