In case you have a secondary device (let's say… iPad) and in case it doesn't have 3G (let's say you were cheap and purchased the WiFi one…), and let's say you still want it to browse the internet while outside…
What do you do then?
You connect it to your iPhone through TetherMe or MyWi
Internet tethering allows you to use the iPhone as a modem and share the Internet connection with laptop. It’s a great feature come with iPhone. However, for certain reasons, not every carrier supports the tethering feature. If your carrier does not support tethering for iPhone, that’s another good reason for jailbreaking.
So go, look for TetherMe or MyWi over at Cydia. Don't forget to say I sent you!