I've had this same discussion with my brother about books. I would read my favorite books over and over and over again; he simply could not understand why, because as far as he was concerned, you read a book once, and then you never read it again because you already know what happens. And I would always retort with "That's not the POINT" and we'd go back and forth like this forever. XD
Same with games. There are some games that I love so much that I can play them a million times and never get sick of them. There's a satisfaction in knowing a game world like the back of your hand. However, that's also something I kind of fear; I don't like the idea of knowing a game so well that it becomes routine. When you stop having fun with a game, it stops being a game.
Buuuut that doesn't really prevent me from playing my favorite games over and over again anyway.

Replay value is a huge deal for me; I love game worlds that are huge and sprawling with lots of side quests, because I never find everything on the first time through, so that pretty much guarantees replay value. I'm currently going through the Shadow Hearts series again with a fine-tooth comb to discover all the things I missed. :3
I usually let a good chunk of time pass between playthroughs, though with some games (God Hand, Drakengard, Okami, The World Ends With You, etc.) I will at least start up a new game immediately after finishing just to play around with what I unlocked. Very rarely do these new games actually lead to a full playthrough.