Bimmerpost is probably part of an ad network that sells real estate (jargon for ad placements) to their clients. Clients can range from anything, but the key is targeting the most relevant ad to maximize exposure. Since this is a car forum, it makes sense. Prolly targeted to a certain demographic who are interested in automobiles.
If you want to get real technical, they could go deeper into targeting ads to you by way of installing a cookie on your computer. That will essentially learn your behavior in which websites you visit and sorta builds a profile of your interests.
Yea it sounds stalkerish but that's the jist of it.