Some small goodies it adds: a bit of additional info accessible in the game (for instance you can check your recovery times with a right click on your Attack/Shoot bonuses), a quick save, an always run feature, a double speed feature (good for traversing the vast landscape fast), a possibility to play the music from MP3 files in a given folder (instead of the CD playback which often just won't work nowadays) etc. It also makes it possible to switch controls, it can add some more flying controls (like climbing/descending with your mouse wheel — these are really invaluable for people like me who play on laptop keyboards that have the traditional "flying keys" all over the place) and it can even add mouselook.

(It comes with quite a large configuration file where you can enable, disable and otherwise set all those things up, so you may need to do a bit of reading first.)