Basically, when video is encoded and compressed for dvd you follow a bit budget based on what is included on the disc (audio tracks, subs, bonus, motion menus, etc). The more crap they want to cram on the disc, the lower bitrate you can run the feature video so it will fit. Superbit just lets removes the bloat so the video can be encoded at a higher bit rate so it will look better.
Yea, I work in the dvd industry lol.