FWIW those rims are called "coining" but all of the desert racers out there will call it "dimpling." They're usually done for two reasons, either to strengthen a sheet with holes in it or as a traction aid. In this case I'd say probably both reasons.
It is quite remarkable how much stiffer a flat sheet of holes will get when the holes are "dimpled." Somewhere out there is a Utoob video demonstration of this effect. Though there are dedicated, and sometimes quite expensive, dies made for coining/dimpling a friend uses old bearing race matched sets with a draw bolt to achieve the same thing.