As a developer, jailbreaking is very important. My only device for a long time was a 3GS and I always wanted to keep it up-to-date when new versions of iOS were released. However, upgrading the iOS is a one-way street -- Apple do not let you downgrade your software.

By jailbreaking the device, Cydia automatically stores the device's unique SHSH blobs that allow you to downgrade.

Being able to downgrade made it easier to test my apps on iOS 3 and iOS 4 without needing two devices.