In an interview Robert Scoble asked Jay Freeman, why should we jailbreak our iPhone? Jay Freeman replied that jailbreak makes it easy to customize and add different third party tweaks to iPhone. He shows off a couple of tweaks such as SBSettings . SBSettings provides a way to quickly access settings using a similar gesture.
Jay Saurik Freeman also shows off Barrel which brings 3D effects to iPhone, GraviBoard which allows you to set gravity of your SpringBoard, Iconoclasm for icons layout, and Springtimizer which allows you to change the slide to unlock text and more. You can watch the video embedded below which shows why you should jailbreak your iPhone.