Just saw an interesting article. According to the register.co.uk:
"Spam comments deposited using the technique include a link to a spamvertised website. The idea is not so much to get visitors to a spammed location to visit the promoted site but to increase its page rankings in Google. This, in turn, increases the prominence of the site in certain searches and therefore increases the likelihood that more visitors will visit a site. That's the theory anyway."
You can read more here: