Why do people spam Web site guest books?

I have a Web site for high school level math that was set up under Yahoo/Geocities. It gets a modest amount of traffic. I have a guest book on it. Geocities notifies me when I get a new entry in it, which I used to look forward to, since entries are not made that often. From time to time someone spams my guest book with pointers to commercial Web sites. Since I have to approve of each entry, they never actually get into the guest book, but it can be a pain having to individually diapprove of each entry. Lately someone has been spamming at a furious rate. If this keeps up I will have to remove the guest book. Is that the objective of the spammer?