I Jailbroke my iPhone for 2 main reasons. One, I needed to use the tethering. When I am out and about in Japan and I don't have a landline available I can use my phone to tether to my laptop/netbook. Two, I wanted to unlock the phone. Carrier unlocking lets you drop in another SIM. I wanted to test this out in Asia as a new SIM cost a lot less than my carrier's international roaming plans.