Like BP says, to offer a more level shooting plane while the shotgun is actually pointing up for a higher POI. Higher ribs allow for a more upright head position in your stock mount. Lessens heat waves in your viewing plane while shooting. The high adjustable ribs allow for a POI change without changing the stock and then to set the back of the rib for personal preference in how you would like to see down the rib.
All these advantages could apply to field, skeet, and sporting guns. With the exception of a rib and stock based, built in higher POI. Why the industry doesn't increase rib height on all shotguns is the real question. Seems like a no brainer to me, and a great marketing tool. Remember when we traded our fixed choke shotguns for the choke tube models? Just had to have the versatility of changing chokes for different applications.
Now we have add-on ribs to raise the over all rib height. KeenSight comany will custom build a stick-on rib to add to any shotgun, any width, any taper. Even an adjustable add-on rib that allows for the rib to be positioned to suite the user.