When you submit a comment to a blog, you usually get to include a link to a website along with the comment. Usually the commentator's name is linked to that website in the resulting comment page. If my spam comment is good, a very small number of people will click my username and go to the site I choose. The more comments I get published, the more clicks I get on my link. If that is a spam link, I get paid for every one of those clicks.

Or, a spammer might generate some good comments just to get the comment classifier to mis-classify a blatantly spammy comment that will be submitted later.