They're typically people hired to post spam on multiple wikis running mostly-automated spam clients, I see the same ads and even some of the same user names on another wiki I edit. I'm not entirely sure how the business side of it actually works, but the general idea is to get the sites they link to higher in search engine rankings.
http://en.wikipedia.org/wiki/Forum_spam
The other reason is the same logic as regular junk mail; if you send out 500,000 letters from your printing press, you only need something like one person in fifty thousand to buy Mexican pharmaceuticals or expired Latvarian army surplus morphine or a commemorative something on an installment plan that works out ten times more expensive than it would be in a store to make a profit. This is just even cheaper since all you're really paying to do is keep a computer running and someone to glance at it from time to time to make sure it's doing its thing, so they really don't care what their miss to hit rate is.