The dual batteries on the Jeep allows you to have plenty of cranking amps for your engine, and also have the right amount of power for all of the accessories that are on your vehicle. The dual batteries allow you to operate your winch and other accessories without draining the power the engine needs to start and function.
Basically, if your Jeep has no accessories then you can do without the dual batteries. If you do have any accessories like extra lighting or a winch then the second battery can keep you from being stranded one day.