1 of 16Image
Because it gives you choice and extreme customisation, and "breaks the device out of its jail" to allow you to install any application.
Jailbreaking, in a nutshell, gives you all of the glamour of owning an iPhone, while retaining the flexibility features of Android. The process itself is quick and easy, and is legal under U.S. law. It rarely has any effect on the App Store applications you have downloaded, but gives you an additional store ("Cydia") to install free or even paid-for apps to make your iPhone experience even better.
In short: you gain so much more, and lose nothing. Well, except maybe your warranty.
Image source: Flickr.