What he said.
You can also play games (though if you want to have a videocard powerful enough for it you will have to deal with more noise).
The fact that you can store all your DVDs on it is a great thing! Think about it, all your movies at the reach of a button. Also the advantage of a HTPC is that you can upgrade parts as you need. You have total control on how the system looks on the screen (skins), as well as how it behaves. For tweakers and fidgeters this is a dream machine!