Switching the default apps. Sideloading. Arranging my home screens any way that I want.Tweaking the system any way that I want. Tweaking the UI any way that I want.
Allowing me to build some software and install it on the device I own without having to get it blessed by Apple.
Allowing me to build some software and install it on the device I own without having to get it blessed by Apple.
These are all very good reasons to jailbreak.