To help disperse the weight of vehicle and passengers, and all the luggage they have.
Well, when you load one up with 55 190 pound (average) bodies and put a big diesel mill to push it down the road, you need to distribute the load over an extra set of wheels, thus the extra axle.
Not ALL do. But for the most part, it is for the weight factor. Not all that have two rear axles are both drivers. Some have one driver and the other is referred to as a TAG (tag along) axle.
