Being able to install and run "non Apple approved" applications. There are thousands of free applications available for an iPhone after jailbreaking.
Tethering allows you to use the iPhone as a modem connected to a PC.
Unlocking allows you to use a non-AT&T sim.
Even though one can use AT&T while traveling, I like to use a prepaid sim from the country I'm visiting.
I have a local number that is much less expensive than AT&T's international rates.

