Old Ways,
Mr Worrell explained it pretty, well You should have a breather on one valve cover and the PCV Valve on the other which is usually routed to the base of the Carb. The way it is designed , if all works right , is it draws ambient air in the breather , down thru the valve cover into the crankcase and then the oil vapors are drawn up the opposite valve cover thru the PCV Valve and then burned in the engine and as he stated it was a pollution thing, the PCV Valve is actually a check valve, and since it has oil vapors being pulled thru it they do need periodic maintainance ( cleaning in solvent ) or even replacement. one thing you should not do ,is not have a way for the engine to vent , ie, no breathers as the faster the engine runs the more crankcase pressure it develops due to the crank sloping in the oil at higher RPM's, if this does happen it'll develop enough pressure that somethings got to give and it's usually the weakest gasket or gasket's. It'll blow them out and the oil will follow. usually lot's of it too ! Hope this helps.