jailbreaking your ipod would allow you to run unsigned code, many developers make software that the app store wont allow, additional you can get almost every app store app for free.
if u have an ipod touch 2g dont jailbreak it i tried and tried but every time i did it it started lagging like a mofo 2 days later
someone should figure out a way to let u downgrade ur shit back to 3.1.2 so it dusnt lagg