Unless they are really dumb, spammers don't register accounts because they would have to give up working e-mail addresses. They usually initiate spamming by posting a simple comment like "Hey, this site is very cool" or some meaningless message like "jkhse" to see if the moderation function is on. More importantly, if you post your URL here, their spiders will put it in their spam lists.
And moshu is right about registering an account. The registration requirement is meaningless for anyone's benefit.
>what is it for?
If you register an account, you can post comments with the same name instead of having to come up with a name every time you post a comment. That's all.