Basically for use to keep the rear of the truck at normal ride height when carrying a heavy load. They do not increase load capacity.
Load capacity is based on many factors beyond just the springs...axles, brakes, frame, transmission...and more are all taken into consideration for load capacity by manufacturers.
Air bags can stiffen the ride when inflated and some folks report their truck has a better ride when loaded with the air bags inflated.
Personally, I have had two trucks with overload springs and my current truck has a Link Air Ride suspension in the rear. All three trucks, in my opinion, were equal ride with a heavy load. The Link system does keep the rear of the truck as close as possible to normal ride height. The overload springs did not kick in until the rear lowered as weight exceeded the capacity of the normal springs to keep the rear up.
