Why would you want to jailbreak your phone?
Why You Might Want to Jailbreak Your Phone Jailbreaking lets you do everything from customizing the look of your iPhone to installing third-party applications, which are titles that are not authorized and available in the App Store. A third-party app can add tons of functionality to your phone that you'd otherwise never see through the App Store.
Do jailbroken apps still work after an iOS update?
Jailbroken apps are not always supported in new versions of the Apple iOS, and each system update pushed out by Apple usually will erase jailbreaks and apps that depend on them. You'll have to jailbreak the device again, reinstall the unauthorized apps and hope they all still work.
What are the pros and cons of jailbreaking an iPhone?
Pro: Access the iOS file system. Jailbreaking exposes the inner workings of an iPhone, iPad or iPod Touch to its user, which can be very useful. For example, installing a secure shell (SSH) client lets you securely move data between two devices on a network.
What can you do with a jailbroken iPhone?
If your device is jailbroken, you can install apps that aren't available from the iTunes App Store. For example, Cydia is a framework that lets you add other unauthorized apps, such as games and networking tools. "Apple is fairly restrictive.