If you're like most WoW
Personally, it served my World of Warcraft fix as I used it to inspect my dream outfits and as a reference for some artwork. It's also good for a few juvenile laughs, like putting dresses on male Goblins. Yeah, I'm kind of retarded that way. At any rate, because WoW Model Viewer doesn't require Internet connectivity to work, it serves as a perfect antidote to a severe case of Internet -- or WoW -- withdrawal. While not strictly an AddOn, I can't recommend WoW Model Viewer enough to players who have the game installed.
There's no real reason not to download it, actually, and clever users like the folks over at MMO Champion even get to dig up some surprise nuggets every now and then. Coupled with a resource like wowhead, you can plan out your gear or at least be prepared for how brutally ugly you'll look in the future. For myself, I can't thank WoW Model Viewer enough for providing me enough WoW-related entertainment until I finally, actually got to log back into the game. How have you (ab)used WoW Model Viewer?