Just to clarify, as far as I know STW don’t sell ads on a singular basis. Like most websites of this type (and I also work in this industry) the ads are handled by programmatic exchanges. Essentially STW pass their collateral onto programmatic exchanges whereby advertisers bid for the right to occupy the available space, based on content, user type etc. It’s all automated and STW have very little say into the add that actually run (though I believe that they would be able to ban certain types or class of ads, though the process would be time consuming).
The reason websites like STW do this is that the alternative (salaried salespeople flogging ads) isn’t cost effective. So the ads that run are the ones that bid the highest for your attention. As I understand it some aspects of it can be tailored to your particular browsing history, particularly if you’re permanently signed into Google or facebook.
I’ve said it before and I’ll say it again. The fact that there are people out there who are willing to derive such benefit from a forum such as this and yet are unwilling to cough up a couple of quid A MONTH to support it is shameful. It reflects far worse on you than it does on a business like STW which occasionally has to make commercial accommodations in order to keep the lights on