They allow you to basically adjust the right height and 'spring rate'. The higher the pressure, the stiffer the suspension. If you have a heavy load, you increase the pressure to carry it safely, and without bottoming out the back end.
Drag racers love them because it allows them to lower the pressure for better weight transfer. Then, they can jump back on the street and suffer no ill effects of a drag racing suspension.
They are also used extensively by ricers and show cars to dramatically change the ride height. Kinda dumb, because unless you have a solid axle, changing the ride hieght destroys the alignment.