The Positive Crankcase Ventilation (PCV) system returns combustible gases which have leaked past the piston rings into the crankcase, back through the intake manifold for reburning. This leakage is the normal result of the necessary working clearance of the piston rings. If these gases were to remain in the crankcase they would react with the oil to form sludge. Since there is also a certain amount of unburned fuel in the gases, dilution of the oil would occur. Both sludge and diluted oil will accelerate the wear of the engine.
Along with gases returning to the intake manifold, the PCV valve also returns a certain amount of additional air. The carburetor used with this system has been calibrated to compensate for the additional air intake.