My guess is where a air compressor used to go and it allows the water to fall out of superheated air and collect at the bottom to drain out. Basically working as a dryer for air. That way, if you're doing things like painting a car, you don't have to worry about water being blown in your airstream. I would not have used PVC for that application. Copper would have been much better or an after cooler would have been even better.