I read on article on this once (don't remember where) and it showed that the popups got very few clicks but they did increase traffic/brand awareness/etc by essentially subliminal means. They want you to just close it without consideration just to plant their name in your head so when you are shopping for a product service they offer you recognize their name without knowing why you recognize the name. If you recognize a brand you are more likely to consider what they have to offer more seriously than a brand you have never heard of, especially when the recognition is a vague recollection you can't place, it makes it feel more important.

The numbers in the article were fairly clear, despite abysmal clicks they would see results.