About half of the posted information is false, and about half is actual fact. Some dealers will suggest adding bags in order to make a sale. The bags do not change how much weight your truck can carry.
Bags can change how your truck rides and they can ensure that your headlights are aimed correctly. I have an on-board compressor so I can change the amount of air in my bags while going down the road, which can effect my ride for certain road conditions. I can also lower my height about an inch if need be by running my bag pressure down to about 5 or 10psi while towing.