From what I've seen, for most people it's a power exchange situation. For the guys, they get the sense of power of "sowing their wild oats" quite literally. A major portion of the men into it want nothing to do with the kid. For the women, I'm not so sure, but there's the bad boy appeal and the risk factor to consider. Some women also see it as a form of financial stability with welfare, child support, and other government programs meeting their needs.
Then there are the people who just want to have as many kids as possible, both men and women. Some do it through the auspices of monogamous marriage,
some do it through polygamist relationships
while others simply want the allegedly dubious status of having as many children as possible by as many women as possible to demonstrate their virility.