There are many reasons for jailbreaking your iPhone, be it video recording, run apps in the background, or customize the theme of the springboard (the list goes on and on). Personally, if I had to choose just ONE reason to jailbreak, it would be the ability to tether my laptop. Tethering will allow my laptop to go online wirelessly through the high speed network of the iPhone. Just imagine the possibilities. Surf the web on your netbook/laptop even when there are no wireless networks nearby or if the networks are password protected.
This brings me to PdaNet,a free and simple app that will make your iPhone act like a wireless router. The beauty of it all lies in the fact that it uses an ad-hoc wi-fi network that allows the iPhone to connect to a PC or MAC. No need to enter any IP numbers or deal with any SOCKS proxy connections.