They increase the gearing range a little, but it's more for MPG than anything else. By comparison, the transfer case on my truck is a 2.64:1 ratio.
Comparing a F150 10spd 3.73 to my Tundra, the overall gearing is:
F150: 1st = 17.5
Tundra: 1st = 14.3, 1st Lo = 37.8
So 1st on the F150 is...