The PCV (positive crankcase ventalation) valve's job is to do exactly that. It maintains a constant air flow through the engine crankcase. This air flow helps purge out flamable vapors in the crankcase that come from air/fuel mixture that gets past the rings during compresion, as well as the vapors emmited by the oil while it is at operating temperature. The positive airflow also carries away combustion blow by which reduces the rate of oil contamination extending the life of the oil.
The PCV valve offers little performance gains, however, the slight low presure it maintains in the crankcase can proportionally increases the pressure differential across the piston rings which helps them improve their seal.
Before the days of PCV valves, the engine's crankcase "breather" was vented to the outside air. The mixture of un-burnt fuel, combustion blow by and oil vapors makes for a nasty concoction which is toxic and harmful to your health. The PCV valve and associated plumbing takes this mixture and pipes it back into the induction system where it is sucked in a burned with the fuel inside the engine.
