Ok so now we've got some responses. Let me attempt to categorize reasons:
1) Limited availability.
-This is valid to a certain extend. I haven't seen this issue in a long time. Even when I bought MW2 and Starcraft 2 at launch day (not preordered) I found copies easily. And if you're on PC there's even less reason for this with all the digital distribution sites.
2) For bonus in game items.
-I don't like this one at all. In conjunction with some other reasons to follow, I suppose it's a nice bonus. But for this to be a reason on it's own is silly.
3) I know I'll like it/I like the company.
-This is the worst reason, really. Ok, ok, yes maybe you really do know you'll like it because you saw some preview videos and it's simply a genre that you feel can't go wrong or whatever. But this is the exact reason Dragon Age 2's metascore by users is sooooo low. A LOT of people thought "Oh it's BioWare!" "Oh, it's DA2!" "Of course it'll be awesome!" and they were sorely mistaken. Not everyone! But many were, and there's nothing they can do about it. Can't exactly get their money back.
Don't get me wrong, sometimes these reasons are justifiable. But, hey, you wouldn't buy a car without a test drive, right? You would marry a girl without first meeting her, right? So why would you spend your hard cash without first playing, or at least seeing first hand footage, of a game?
Also, as we've seen lately, you can't rely on the preview material. You can't trust the promotional interviews. Of course they're going to focus on their checklist of great features, but they might miss out on some horrible design decisions that won't come to light until after you already have it in your hands.