[RichardJ]

The breather cap on the valve cover is just a chunk of steel wool. Taking air from the air cleaner gives you cleaner air going into the crankcase. Steel wool only stops the big chunks.
During high load or WOT the PCV usually can't pull in all the blow-by, even on a perfectly health near new engine. The excess fumes go out the breather into the atmophere. When you have the connection to the air cleaner, the excess fumes go in above the carburetor and are consumed by the engine.

The later is called a Closed PCV System and is the only type of PCV System that has been legal for the last 50 years.

Check to see if you have a problem with your PCV valve. take the cap off and place the palm of your hand over the fill hole. You should feel a small amount of vacuum at idle. Pull the valve out and with the engine running put your finger over the hole in the valve. You should feel plenty of vacuum and you will hear the pindle snap to the wide open position. If you take it off, you should be able to blow air through it from the valve cover side, but not the vacuum source side. It should close up from that side if you have a back-fire in the carb or manifold.

The PCV valve is listed as a regular maintenance item on all cars. It is supposed to be cleaned or checked almost as often as you change your oil. NO one does and they get gummed up.