If a game I want has a Collector's Edition/Special Edition/Limited Edition, I will buy it. There's very few cases where I'll pass up on a CE/LE/SE, but I would say that I buy them for 95% of the games that I get per year.
They usually always have something that I want in them. A poster, a wall scroll, figurines, a soundtrack CD... stuff like that. Occasionally they are also the only way to get a physical copy of a game, and I will always get a physical copy over buying digital when I can.