Like JFreak said, fullscreen and ease of saving things from a browser are nice plusses. The big ones for me, though, are the lack of the splash screen when you start QT Player (which I actually like), and the MASSIVE variety of basic editing and recompression options that it opens up. You can pay hundreds of dollars for access to a compression system like the one QT Pro has.
If you work with audio, there's less reason, but you can still extract audio tracks from a video file, cut and paste audio clips (possible in iMovie, but easier in QT Player), and export to a variety of compressed and streaming formats (not MP3, unfortunately, but almost anything else).