I realize that hacking your iPad voids the warranty so before I buy an extended one I'd like to know the
upside to doing it. What can you do after jailbreaking that you couldn't do before?
Yes, jailbreaking voids your warrenty - while the jail break is on the iPad. However, it can be easily removed, thus restoring your iPad to stock/out-of-the-box state. This means that, should you need warranty work, you can remove any/all jail break software and none will be the wiser.
A question I asked before, still no answer - will jail breaking make your iPad less secure than before, i.e. will would-be hackers/viruses/worms now find it easier to mess with your Ipad?