I sell ads for a living, and yes people do click on them. The click through rate is very low on most sites, but enough people click to make it worthwhile. The more trusted the site is, the better the clickthrough rate is. For instance, you might have clicked on the ocuk store banner on the forum header (provided you haven't blocked it), because you trust it. It's unlikely that you'd click a banner on a pr0n site though. Similarly, people tend to trust ads like google sponsored results, so they are worth buying.