I think the main reason is for when registering a pickup. At least some states charge additional fees based on the GVWR of the truck. A 3/4 ton's registration fee is slightly less than a 1 ton.
I have a 3/4 ton, as that's what was available in the configuration I wanted when I was searching for a slightly used truck years back, but I later found out that I have the exact same axles and spring packs on my 3/4 ton as the 1 ton SRW had in that model year. It was something Ford did for a while (not sure about now) in the interest of cutting costs - instead of stocking 2 different sets of springs, axles, etc, they put the same components on both vehicles. The ONLY difference between the F250 & F350 SRW driveline & suspension was the F250 had 2" rear spring blocks, and the F350 had 4" rear spring blocks. My truck even has the longer U-bolts that will fit the 4" blocks.
So, aside from the blocks, I have a 1 ton suspension with the 3/4 ton sticker on the door, and slightly lower annual registration fees.