Every engine made after 1966 has to have a PCV in the valve cover. In 1968 they changed the block casting to remove the old road draft tube that older cars had. The PCV is a one way valve (only allows vacuum not pressure from an explosion such as a back fire) connected by means of a rubber hose to your carb or manifold to get a strong vacuum. It evacuates all of the oil fumes from the motor and burns them to cut down on hydrocarbon emmisions. In order to bring the fumes out of the crank case it has to have fresh air entering the engine. There used to be an oil fill tube with a filter on the top of the tube; but that was replaced by another hose that goes from the opposite side valve cover of the PCV to the air cleaner housing. In side of the air cleaner housing there is a plastic fiber filter that keeps bugs and leaves from being sucked into your engine (lets in all the dirt and grit you can though, as the factory wants to sell you a new motor at about 100,000 miles).
When you go to an open element you loose the air cleaner housing filter and the hose and have to replace them with something else. I gave you the two options that I know of above with pictures of them in place.
Without a functional PCV you will have leaking gaskets and lots of blow by as the slightly lower air pressure in the crank case actually helps the rings to seal (a mechanical vacuum pump will make you 2-3% of your crankshaft horse power as a bolt on accessory because the rings seal better with a vacuum in the crank case).