What the nofollow does is tell search engines not to follow (or score) the links in the comments. More here: http://en.wikipedia.org/wiki/Nofollow. It was created primarily for the purpose of addressing the comment spam issue.

According to the wiki article, Google, yahoo, and bing all pay attention to this.

So leaving comment spam SHOULD NOT help the spammer’s pagerank.

So I’m back to what the heck are they getting out of it! :)

So if you can spam thousands of blogs with a link back to your site, the search engines simply read that as “thousands of sites are linking there” and will increase its pagerank accordingly. The goal is to get the site up in the first page or two of search results, where it’s more likely to be clicked on.

Incidentally, even though it’s not being maintained by the author any more, I love Spam Karma 2 for WordPress. It’s really very good at recognizing spam comments, and you don’t have to authenticate against the WordPress site the way you do with Akismet.