What is the purpose of dual rear tires?

Answer 1

One purpose of dual rear tires is to distribute very heavy loads over a larger area. Though tires can often handle the load, they will often sink in on soft ground.

Another major advantage is that if one tire has a blowout (suddenly loses its air), the other tire will take the load and the driver won't have trouble controlling the vehicle, sometimes even able to continue driving to a service shop.