The iPhone is a powerful device but with less distractions, animations, etc... The lack of those thing in the iPhone make the iPhone more faster more powerful and more reliable, most of all, the fact that rooting is lock prevent the instrusion of malwares viruses in the iPhone. Basically it's almost impossible for a virus to get into the iPhone since Bluetooth is extremely limited in function, syncing files from your computer to the iPhone have to be made all via iTunes which seriously limited the risk of syncing viruses to the phone. Compare it to the droids and symbian OS, you can root on them the way you want and the distractions on those phones are huge say theming, ui costomization, etc... Resulting of a low response when come the time to use the phone functionalities in the phone, drop of calls are very comon on those OS, a lot of serious bugs I'm not going to name here otherwise... And the way those OS symbian or droid can easily catch viruses malwares and so on.
Regarding the competition on the market, apple tag to make secure iPhones, reliable iPhones, and sadly the only way to archive that goal is to load tons of restrictions on the iPhone. Jailbreak comes to break down those restrictions impose on the iPhone by apple, that said, jailbreak widely remove and open the hole iOS main securities entities which lead to you been able to load whatever you want on the device and by pass whatever security apple impose on the device. You can avoid iTunes on you computer and still load remove songs videos files from your device using 3rd party apps on your computer like DiskAid or right from your device by using tools like iFile or celest Bluetooth, etc...
And ofcourse like on the droid, the more you loading thing on the phone say, theming, navigations, systemes, you are capable of messing up the device if you don't know what you doing which will result you having a brick iPhone at the end. Some rare iPhone viruses are capable of destroying the hardware of the iPhone. In addition things you are installing on the device from Cydia after jailbreaking can decrease your phone performances in all aspects possible, depending on what you loading on the device and where you get those apps/tweaks from. Apple is award about all this and so refuse to allow jailbreaking the iPhone since it will go against his goal of security, reliability and speed.
