The reason for jailbreaking your iPhone is pure and simple, to have the ability to install 3rd party apps not sold in the App Store. I know there are thousands of apps available in the app store, but apple has been very restrictive with their rules regarding which apps can be made available to users.
Apple, in their infinite wisdom, has locked down the iPhone in a number of ways, hampering its ability to do basic tasks like record video, zoom in when taking photos, or apply a theme to your homescreen. They may have been right to do so – many cell phone owners have had devices with this functionality for years, but could never understand how to access those functions.
Therefore, if you want more from iPhone 4, go ahead and jailbreak it.