You legally have to display a license plate on the back of your Jeep if you drive it on public roadways, but if you've modified your Jeep, and you can't keep your license plate in the factory location, then an aftermarket license plate bracket is the solution for you. A popular option is to use a bracket that mounts your plate in the middle of the spare tire. By moving the license plate here, you no longer have to worry about damaging it when you're out on the trail.
If you have a 2007 and newer Wrangler, and you've already moved your license plate to a new location, there are delete plates to cover the holes left in the rear quarter panel from the original license plate bracket.