Pre-ordering games often comes with nifty bonuses. For instance, here in Canada pre-ordering through gamestop allows me to receive one of 3 random posters with the game. It's not a huge bonus but it's still extra free stuff.
I don't know how it works everywhere, but whenever I pre-order a game through gamestop, it only allows me to pay a small deposit of $5 after which the remaining balance is paid at pickup. Which I'm fine with but it also means I need to continue budgeting until the game's release. Sometimes I like to prepay for things for budgeting purposes.