This hole allows the air to go directly over the oils. Without air going over the oils, the concentrates will not vaporizer well, or may not vaporizer at all. Without the carb cap, the air goes over the oils and does not fully hit the oils to allow them to vaporize.
With the carb cap on top of a banger the hole also allows the user to breathe. Without the hole the rig, banger and carb cap would be blocked.