1. Expecting that the game will be pretty good (mostly the hype does this job), or knowing that it will be a sequel or remake of a very good game.
2. At amazon, because I don't live in the US and that's the only place where I've bought so far and the prices are good. Haven't really tried anything else nor compared it to others to I can't say if it has the best prices. Also I like that if you pre-order at amazon, and the price decreases they give you the lowest price.
3. It is good to know that you'll have your copy of a game just as it launches.