My Main Reason for Jailbreaking: RetinaPad.
There's literally just three reasons why I Jailbreak my beloved iPad. One is the infinitely handy SBSettings, another is for the Last.Fm app Scrobbl. But my number one reason for Jailbreaking has to be the absolutely awesome RetinaPad. I mean, just look at the difference it makes to iPhone/iPod touch apps!
What you see there is a knight from the iPhone version of Eternal Legacy running on an iPad. On the right he's being rendered with the iPad's built-in pixel-doubling technique - not pretty. On the left, however, he's being displayed via RetinaPad. Instead of pixel doubling an app running under RetinaPad get's rendered in the way it would do on an iPhone 4 - displaying at 960 × 640 px rather than 480 × 320 px. And boy do the results look great! Of course it won't work for every app, just the Retina compatible ones, but for those that it does there's incredible improvement.
RetinaPad is definitely a must have for me, you can find it on Cydia for $2.99.