What does setting Media Kind to Audiobook do?
Jesse Hollington wrote a great guide to Audiobook handling "The Complete Guide to iTunes Books, Podcasts + iTunes U" at iLounge. Basically it comes down to separating AudioBooks from music and providing different sync and management options within iTunes.

Your sync differences are obvious, you get a different sync category enabling you to sync all or selected Audiobooks. One really interesting difference is that on iOS devices, media kind Audiobook lets you adjust playback speed. I haven't tried setting a music track to audiobook and then changing playback speed - might be useful for learning guitar tracks if pitch is maintained.