Majority of ads (through adsense) are contextual so yeah you might get some weird ads in some threads (I remember a very manly thread which got derailed pretty quickly because people where getting very "manly" ads in it

)
Then you get your interest based which are tailored on the persons history (that's why if you visited such or such online shop you might even a couple of months later get ads about them, I still see ads from an electronics shop I bought from 6 months ago and which I never had any ads before)
Finally you get your placement ads which the ads buyer decided to target your website specifically
In the end the ads with the best price are the ones which are prioritized. And you might think nobody clicks on ads but the numbers are between 0.8-2 % clicks which isn't that bad considering that average payout can be 0.14-0.20 € a click (RPM or revenue per thousand impressions is a bit different but is so rare now that its becomes meaningless)
So in the end for people who don't have much cash or don't have a credit card you now know that you can help out your favorite websites by paying a bit more attention to the ads they support

(and I don't mean randomly clicking on them and leaving a few seconds later as that tends count as null or done too much can devalue a websites ads worth)