This is the method i have found the easiest and best. It unlocks, activates and jailbreaks your iphone all through a user interface that is easy to use. Just follow the guide.
Why would you want to do this to your iphone?
1. if you live in a country where iphone isn't sold with a plan and want to use another SIM card other than official iphone mobile service provider
2. you want to install 'unofficial' apps and get more access to the iphone's os x operating system and features