BTW there are solid ribs, also.
I've gotten serious mirage on an old solid rib side-by-side. However, I have not had the problem on an old 1100 without any rib at all, just a round barrel and a bead on a post.
The rib is there to draw your eye to the right spot -- subconsciously. If I "sight" a shotgun, I miss. I have to focus on the moving target; the rib helps naturally point the gun by letting my peripheral vision pick up the barrel (sort of backwards of a rifle, where you focus on the sight and pick up the target).
The ventilation is to keep higher ribs light, and to help prevent mirage. (Also, per mswestfall above, raised ribs serve the further purpose of allowing the manufacturer, and sometimes the shooter, to adjust the point of impact).