guestbook spammers: seriously, what is the point?

OK, I understand people wanting to spam a guestbook with their links to their look-at-hot-naked-teenagers-while-gambling-for-fake-Rolexes-and-V1agggggg_r@ sites; they want to get their link out there. But I get an average of about 3 a day where they just put "Preview" in every field. WHYYYYYY?