Knowing how to shift can help with hauling heavy loads and heavy trailering even when driving a vehicle with an automatic.
I think BIG trucks still mostly have manual transmissions, (I could be wrong) but I'm thinking when running an automatic in a 3/4 or 1 ton pickup running near capacity on mountain roads. On the flat and level not so much, but climbing in and out of Salt River Canyon hauling a full size 3/4 ton pickup with a full size 3/4 ton pickup I was reminded that knowing how to shift, manage rpm, and use brakes without burning them up was invaluable and the truck I was using only had a 6 speed. Even sometimes climbing over telegraph pass I like to keep an eye on everything and have been known to shift manually instead of trusting the automatic to stay in the right gear for engine braking or descent assist depending on the make.