This is my guess, and if I'm talking ball cocks, then sorry.
My fancy is that it does the same kind of job as an air tank in a compressed air system, compensates for loss in the pipes. Of course forced ventilation is very low pressure, more volume of air. So no need for steel pressure containers, wooden box reinforced do fine.
Alterntively, it could act as a manifold.
Possibly both jobs?
I am sure there is an expert out there somewhere.
My avatar is a poor likeness.