Often you'll see wordpress comments where the comment is some generic praise to make it seem legitimate and avoid deletion, and the "website" field is filled out so their username is a link to wherever they want.
One of my sites gets these every now and then and, given that the website field is omitted on my forum, users are usually quite bemused at the mysterious intent of the authors.
Other responses covered the basics, but there are a few other interesting uses.
- Text that does not clearly market anything or provide a meaningful backlink may be part of a larger link network, or just simply testing to generate a list of vulnerable blogs. Those lists can be sold to others or used for your own purposes.
- Similar to some recent spam bots hitting Google Analytics referrer results, these people may not care about organic rankings. They might be marketing to the blog owner who sees the comment and investigates the username/link. If I'm spamming for SEO services, and you are a blog owner looking to increase traffic for example, the attempt may not be about ranking for SEO terms (a losing battle), but simply getting you to look at my site.
It's pretty much useless now that Google actually penalises people for this - pretty funny that brands who use to do this now have to beg for their spam to be removed.
Basically they would spam these out to not look overly spammy, and have their website and name listed as their website they are trying to promote as people use to believe that having millions of shitty backlinks is good SEO.
It seems pretty counterproductive for Google to penalize the linked-to website for this, because they don't necessarily have any control over the links. This means that now a good strategy would be to make a bunch of spam links to your competitors' websites and have their rankings go down through no fault of their own. While that would be less effective than having your own go up, it's still useful.
What Google should do is make the links have no effect at all, thus preventing this abuse.
This is tricky. Generally the spammers are doing it because they're paid to do it by someone. Penalizing is the right thing to do. But you raise an interesting point - perhaps an unscrupulous firm would unleash a spambot pointing to their competition, to damage them. I'm not sure of a good way for Google to differentiate, other than your suggestion of ignoring.
Thank you for allowing me to talk myself into a circle. :-)
A lot of these guys are all about A/B split testing. If they find something that hurts rather than helps, they'll use that tactic as an offensive on their competition.