When an engine runs, a small amount of combustion gas leaks past the piston rings into the oil pan. This is called blow-by. If left unchecked, these gases build high pressure inside the crankcase. This pressure will force engine oil past the valve cover, oil pan, and main crankshaft seals, creating leaks. The crankcase breather vents these gases out of the engine, routing them back into the intake tract to be burned safely in the cylinders.
The breather typically contains a baffle filter or oil separator. This traps oil mist and lets it drip back into the oil pan, allowing only the gas to vent. If the breather vents clog with carbon deposits, pressure climbs, blowing engine seals and making the engine burn oil.