I used to have the same question - then I set one up for internal documentation. Features like searching, ease of linking, version control, etc. get bandied around a lot but the simple, compelling argument is that they're easy enough to use that people actually use and, most importantly, write documentation.

In most places the general process goes something like this: nag someone for 6 months and they'll either foist the task over to someone lower on the food-chain (who is, of course, completely incapable of doing the job well - that's why they weren't asked in the first place) or eventually produce the bare minimum needed so they can say it's done with a straight face; this will be placed somewhere where a couple people will look at it once or twice and never updated. Eventually someone will realize that there's no documentation which reflects what's actually going on and start nagging someone...

With a wiki we actually have people writing documentation as they go. There's always going to be someone lazy but the barrier for entry is so low that they won't be able to come up with lasting excuses. Basically this is the first time I've worked anywhere where we have up to date, accurate documentation and it's basically due to lowering the barrier to entry that much further.