Why would someone want to risk unlocking/jeailbreaking their phone?

Why would anyone want to risk breaking an iPhone worth few hundred dollars to use on another carrier? They say unlocking any phone allows you to use it on other carriers. Jailbreaking an iPhone allows you to install applications that have not been approved by the App Store (which also have the possibility of causing harm to your phone).

Now, why would someone want to risk buying an expensive phone for that? A better approach to using an iPhone is just signing up with AT&T. The best part of having an iPhone with AT&T is support. Since they are the only carriers for that phone, they are the only ones who can provide network support.

What are your thoughts on the risk involved with unlocking/jailbreaking your iPhone?