Even with the 10 speed. First gear still has to start it all moving, and a lower gear will start it moving much easier than a high gear. That is why Lower geared trucks have a higher tow rating than higher geared trucks. There is a world of difference between a 4.10, rear, and a 3.15 rear. Even in a 10 speed. When I made rear gears. We even had a 2.15 gear. Want that in your 10 speed with 8000lb behind you?
Also my trailer truck had a 13 double over. And 1st gear would only run it to 10 mph at 2500 RPM. That is a pretty low gear, and it had one even lower when needed.. We called it the Granny gear. We never used it. Wasn't worth the time to put the shifter in it.