I think the first use was for high-volume trap and skeet users....supposed to raise the sighting (pointing?) plane above the hot barrel to avoid heat mirage from a hot barrel. When I was a kid in the sixties, most pumps and autoloaders came in the standard version )plain barrel) and deluxe version with a ventilated rib. It seemed to become a "necessity" soon after that. I used to hate the look of a plain barrel, but I now kind of like the clean lines of a plain barrel...but try to find one on a new shotgun!