To watch your iTunes content on your TV.
If you've only got a few music videos and songs, and not a lot else, then it's probably not worth your cash.
But me, I have an expansive library... several movies, several seasons of TV shows converted to h.264, several purchased TV shows, music concerts.. I've been converting as many DVDs as I can to h.264 video and I enjoy seeing it all on my TV without changing discs and shuffling through bulky boxes to find a particular episode I want to watch.
It's really nice to be in bed and to have literally thousands of TV shows and movies just a few clicks away.
I also have an iPhone and will be getting an iPad, so having iTunes on my desktop and all of these devices that all of my media works on is really nice to have.