- changing the iOS experience with powerful tweaks that can't ever exist on the AppStore : SBSettings, WinterBoard, IntelliScreenX, LockInfo, and lot of others...
- playing retro games on console and arcade emulators.
- having access to a unix shell in you pocket, and all the open source software that can run on a unix OS.
The jailbreak is not meant for (and we are totally against it) :
- pirating AppStore applications
Also please don't pirate Cydia tweaks, there are people working hard on these, and the price is usually low. There's no reason to do that.