Well, I don't know what your interests are, but I, for example, have all my music on my computer, but the computer isn't where I generally want to listen to my music. Xbox Media Center gives me access to all my music. It also gives me access to all the pictures stored on my computer, which is nice when I want to show someone a picture who's visiting, but don't want to take them into the office. Lastly (and what I use most) is the video streaming, as I will rip DVDs to DivX files and store them on my PC. In a fit of unethicalityness I got a subscription to Netflix. The disc arrives, I rip it, and send it back. I've got probably 30 movies and quite a few seasons of various TV shows that I haven't seen sitting on my media server. I figure it'll come in handy when I'm impoverished from being in law school and not working anymore and won't be able to afford other forms of entertainment.
For what it's worth, I'm in a fully networked ReplayTV household and I still use Xbox Media Center far more than my Replay (not to mention that Xbox Media Center can also stream from my ReplayTV box, and I think from Tivo boxes as well).