In my case it's almost always so they can take another shot at some point. It's hard these days for women to find guys with the kind of frame control that it takes to have a good relationship - most guys either shoot themselves in the foot on the way in or do it on the way out.
I've also noticed that women who feel like you were too dominant and they couldn't handle you will hang around, maybe partly out of ego and partly out of the sense that if their backup plan of sprucing up some beta doesn't work they might be able to come in and negotiate at a lower price.
Also, there is the fact that when you treat her a certain way and make her feel a certain way that she can't get anywhere else .. well, where else is she going to get it?