They're popular with webmasters, because:

1. If the popup opens and you don't close it, many times it can track where you're going.

2. The popup many times jumps right in the middle of the screen, and hopefully you'll click on it (accidentally) and thus give them a hit or a click to the site that's being advertised

3. It's easier to make a pop up like that and change it to different ads, than to code one directly into the page.

Pop ups are just getting ridiculous...