Tapping PCV hole on MP Perf. Valve Covers..

I think it was part of the CCVS (Closed Crankcase Ventilation System). Earlier systems just used a valve cover breather & a PCV valve but this system had less chance for fumes, etc. getting into the atmosphere. Anything expelled would immediately be drawn into the engine through the air cleaner/carb. Plus, as rumble said, it keeps the inside of the engine cleaner. Most likely a Govt. mandated primitive pollution control system. The older systems would just vent into the engine compartment. It probably also kept the top of the valve cover a bit cleaner. (Not to mention the even older "road draft" tubes. Remember them?)