The only reason I can think of is that the gas tank is on the driver's side, and if the truck is predominantly used by a single person, all the weight is on one side, putting a bit more load on the driver's side bar. I know with a full 30+ gallons of gas (over 200 lbs), and my big 320lb ass in the seat, the truck has a slight lean to the driver's side. That's why you'll often find that the bolts are turned in further on the driver's side, the elasticity of the bar has degraded over the life of the vehicle, and the key has to be turned in further to achieve the same ride height.
Swapping the bars puts the "less degraded" bar on the side that takes the brunt of the weight.
Now that I actually thought about it, I think I'll be doing this when I swap out my keys in a few weeks, I'd have already done it, but I want to wait until the 30 day warranty on the Yukon XL I just bought runs out. Don't want the dealer saying I did anything to the truck, and voiding any issues that might pop up.