Hi Ross;
They are used to direct air more smoothly into the air horn of the carb, when not using an air filter. As they do this, they produce a longer column of moving air which has its own inertia (like a tunnel ram, just on the side before the venturi) and this helps cylinder filling. That's where the term velocity stack comes from I think. With any kind of performance cam they will also contain the "stand-off" which is fuel air mixture expelled from the carb at certain periods of resonance. In short they add a small amount of horsepower, maybe 2-3%, and sound terrific.
I just sold them last week to fellow in Georgia.
Thanks for asking,
Rhys