primarily, the ability to play DVDs...including rental DVDs, instead of having to rip them and then transfer them to your media player. While these forums might give you the opposite impression, doing all that stuff is pretty 'geeky' and most consumers don't want to get their 'hands messy' with all this. They'd much rather get a disc, pop it in a tray, and watch the movie.... unless of course they're into illegally downloading movies.
It's the same reason I don't particularly fancy a computer based transport. I'd rather pop my CDs into a CD player and play them. Yes yes, I know the convenience of quieing up all the music that you own and playing it with a click and all that, somehow that doesn't seem very appealing to me compared to playing a disc, which is also why I am leaning towards a blu-ray player instead of a WDTV Live.