Air must circulate through the crankcase when the engine is running. This removes water, gasoline, and blowby gases from the crankcase. This also helps prevent the formation of sludge. In most engines, crankcase ventilation is provided by the positive crankcase ventilation (PCV) system.
Engines have some blow by. This is mostly unburned gasoline and some products of combustion the get past the piston rings and into the crankcase. The crankcase ventilation system prevents pressure buildup and oil leaks. In addition, it removes blowby gas to prevent the formulation of corrosive acids and sludge.