It's less a matter of flattening the blade as it is removing the factory polish or coating. New blades come very inconsistent on the bottom, but the coating really gums up a sharpening wheel. Removing that first layer with a mostly flat hollow makes the first sharpen and every subsequent one easier. If you try to do a new cut with a normal hollow, at least with a traditional sharpening wheel, you heat the steel and do a number on the wheel unnecessarily. Plus it takes twice the time. I imagine the same principle applies with these sharpening rings.
Also, I would put out there that a "flat" ring and say a 1 1/2" hollow are reasonably similar at those widths, you could accomplish the same with either