The point is you can combine all the things you mentioned plus more, on one attractive AV HTPC case. For dvd watching, you can scale the image to match exactly that of your tv/projector because of the high end video cards available. The picture can blow away most stand alone players in my opinion. For constant height screen setups, you can set the dvd player software to automatically stretch/adjust the image of 2.35 material as a prerequesite for an anamorphic lens and make use of all the pixels of your HD projector.
Lets see what else; Internet/Radio/HDTV/Recording/Streaming trailers/archiving movies and music for instant access/wireless control from your couch of everything.....
I don't know there are so many things....come on Dan/Dennis and others...