Air shocks..why?

This may be a stupid question , but on my recently purchased 71 duster project car I noticed the previous owner installed air shocks on the rear. Has the air valve installed under the bumper.
The car has a fairly done up 340 and many other race mods tho I don't know much history on the car. Seems like it was on its way to becoming a street/strip car.

But why adjustable air shocks? What's the bennifit