In short: reduce wear and tear on the front part of the drive train. The axle shafts are permanently connected to the front wheel hubs. Even with 6x6 disengaged, the front wheels spin the axle shafts and the front drive shaft passively. This sucks power and causes wear on the assembly for no good reason. The lockout hubs stop that. The wheel hubs spin freely and the axle shafts and drive shaft sit still until you lock the hubs.
The air switch only connects the front drive shaft to the transfer case. It cannot help with the front wheels passively spinning the whole assembly.
The lockout hub on ONE side of the forward rear axle serves the same purpose; it causes only the rearmost axle to put power down, for increased "fuel efficiency" (LOL!) and perhaps, maybe a better turn radius. You can also get hung up on a speed hump or banana peel. IMHO, the rear lockout hub is a bit questionable, unless you do a lot of long-distance highway driving.
