You usually find slot heads on 12-fret and (often) on short-scale guitars. The theory is that by putting the string in the slot instead of on the face of the headstock, the increased break angle will have a positive effect on tone. My primary guitar is a slot head 12-fret, and while I'm completely used to changing strings on it, it's never as easy as changing strings on a paddle head.
A lot of people like slot heads for the retro look. I'm not a fan. I ordered my 12-fret one month before Taylor decided to make paddle heads available on custom orders. Had I known, I would have waited and got a paddle head.
This, of course, is only my opinion. Other people love the looks of slots and don't mind the incrementally more-difficult string changes.