It is also one possible cause of expensive oil leaks and sludge buildup in an engine.
All automotive engines are lubricated with oil and when oil is churned by moving parts, pressure is produced.
There is also a small leakage passed piston rings and valve guides, that produces positive pressure in the crankcase. Many years ago, this pressure was simply vented into the atmosphere by a road-draft tube and breather cap.
Today the PCV system handles this task and other things.