Decelerator allows you to customize the acceleration setting of your mouse or trackpad. While System Preferences can change the tracking speed, it doesn't actually allow you to modify the factory default acceleration setting. There are a number of reasons you might want to do this:
1. Gamers often disable mouse acceleration entirely to gain a competitive edge in "twitch" reflexes. With mouse acceleration off, a distance on-screen corresponds with an exact distance your hand must move, and is unaffected by the speed with which it moves there.
2. Windows users often find Mac OS X's relatively high mouse acceleration disorienting and frustrating. Lowering the mouse acceleration can go a long way to easing the transition to a new operating system.
3. Some people prefer higher or lower acceleration settings for many reasons both logical and illogical. If you're one of those people, then Decelerator allows you to tweak to your heart's content.