1) Customization
With the iTouch not jailbroken, the only thing you could customize is the lock screen wallpaper. With it jailbroken, your iPod could look like this:
2) Free Apps
When you jailbreak your iPod, you can get any app you want free!
3) Extra Functionality
A jailbroken iTouch has extra functions that are very helpful, like the dock having multiple pages, for example.
Whether you jailbreak your iTouch or not is up to you, but I highly reccomend it.
My next post will be a tutorial on how to jailbreak your iTouch.