Jailbreaking essentially is/was used to add features to an iDevice, not just apps. Jailbreaking an iDevice also grants the user root access to it, so you can do pretty much whatever your little heart desires. However, jailbreaking has become less useful as iDevices moved forward, and not all jailbreak features were "healthy" for past iDevices. For example, jailbreaking a 2nd Gen iPod Touch could, if you wanted, give it multi-tasking (it did not receive that in the 4.2.1 firmware update). However, this was really stupid because the 2nd gen touch only had 128MB of onboard RAM, and combined with a 620MHz proessor (which, I might add, was UNDERCLOCKED to 533MHz), the touch just couldn't run multi-tasking efficiently. Wasn't going to happen. In addition, most "features" that jailbreaking added are now in the vanilla firmware of modern and even some older iDevices. Some third party apps that Apple doesn't support are useful, but remember, there is a reason Apple does not support them. Besides, the app store has quite a few apps that do similar things to ones that require jailbreaking. I should also add that contrary to popular belief and the claims of jailbreak developers, there is a slight possibility of bricking your iDevice. You have to really screw up for that to happen, but it's possible.
As for modern devices from Apple, jailbreaking is not terribly useful on the iTouch platforms. There were numerous attempts at jailbreaking the 6th Generation Nano (the small square with the touch screen), but after about April of 2011, all websites that supported it went down, all progress disappeared and it remains nothing more than a fading memory. I still, to this day do not know why this happened, and I can't find an answer anywhere. All I know about that is what I've just told you. Feel free to ask me any questions you have; as even though I am not an "expert" jailbreaking is something I'm quite familiar with and I've played around with it a lot in my life. Plus I've posted a similar thread some time ago and I never got any responses lol
