Other than watch movies?
Let's see..... Using Win7 as the example: there's an in-box Netflix client, and an in-box Internet TV client (which has a somewhat decent selection to choose from); an available third-party Hulu client, and others (Google for 'windows 7 media center add-ons').....
It can play music and videos that you have (be it on the HTPC itself, or on another machine on your LAN).....
It can (if you install TV tuner(s)) also be a DVR - and, it works quite well at that job. The TV/DVR functionality can do HD, too, so long as you have tuner(s) that can do it. I've enjoyed HD on both of my boxen (main MC, and my desktop) for about three years now (broadcast channels tuned by my HDHomeruns); my desktop has had an ATI CableCARD tuner for ~1 year now (and, can tune my full cable package natively. SG:U looks amazing in HD.)
The main box had a Ceton InfiniTV4 (4-stream CC tuner) installed about 4 weeks ago; it gets its CableCARD installed this Saturday.
Liking it so far?
And, I operate all of that with a remote control. I hardly ever touch the keyboard for the main box (which sits in the living room these days.)