You can back your contacts, text messages up, and you can sync photos or videos/movies both ways.
You can view movies from your phone on your pc directly, that goes both ways; watch from cell on pc and from pc on cell. I believe you also can download apps from kites onto your cell but that doesn't work anymore because of google's market policies. Apparently, you also can make your cell work as a modem, but I didn't try that so...
Anyways, as edelcom and mru said before me it still needs some work to live up to what it should do and to keep up with handset itself.
Lastly, in samsung defense, kies isn't just for androids, it's their software for the whole cellphones lines, and many users are happy with it, obviously not us, android handsets' owners.