What's the deal with pop-ups?

While visiting certain, uh, websites and trying to close same, numerous pop up windows appear. I understand HOW they work, but what’s the point? It’s usually a window of links to other sites (which will inveriably result in MORE pop ups). Since they are annoying, you start closing old windows only to have new ones open up. If it’s an advertising gimmick, it ain’t workin’ since the window is never open long enough to read anything. Please explain.

They are annoying, but you can get rid of them. There are quite a few Pop Up Killers on the net, but the one I use is called POW! from http://www.analogx.com.