If you have a blog, you've no doubt noticed all these 'comments' appearing. They something generic like "I saw your site. It's cool. Check out mine".
While it would be nice to think these people ACTUALLY came to your blog, in reality, there are some software programs out there that post these automatically to tens or even hundreds of blogs. Why you may ask? Simple. All these comments include a link back to the commenter's site. Search engines then spider their sites, see all the links from external sites and grant them a higher ranking.
One of the biggest advantages of more traffic to your site - especially if its a blog - is the increased opportunity to earn money from Google Adsense. If you own website, you might consider hosting some adsense. A little daily revenue from click throughs doesn't hurt anybody!
Naturally, many of these 'comments' point to sites heavy in Adsense. Some of them provide tips on how to increase your adsense revenue. Most are niche sites that contain content designed to attrack higher paying Google Asense keywords.
There is nothing bad or offensive about these ads from my point of view. After all, visitors to a subject specific site will find ads about that the same subject - possibly adding to the content the website host has provided.