An auxiliary transmission is a second transmission that sits next to the regular transmission and allows the vehicle to haul heavier loads and adapt to different uses. An auxiliary transmission allows you to access more gears, which means you have a chance to get more of exactly what you need out of your engine.
Having an auxiliary transmission allows you to gear split. Not all vehicles need auxiliary transmission, but they are very good for vehicles that race, haul, climb a lot of hills, or serve dual purposes, such as a work truck that occasionally hauls a trailer but also needs to drive at highway speeds without guzzling a lot of gas.
Due to the amount of fuel you can save by using an auxiliary transmission for some types of vehicles, they can be seen as an investment that pays for itself after driving so many miles. If an auxiliary or other type of transmission fails, having a semi-truck roadside assistance plan can get a truck driver back on the road much more quickly.