As a pollution control system, many cars for a very long time have had recirc systems of valve cover air to separate oil vapors from it, to lessen the impact of those vapors entering the engine and being burnt and sent out the exhaust system.
Burnt oil is horribly bad for the environment.
When this recirc/separation system fails, it fails to remove the oil and oil vapors from crankcase atmosphere and subsequently from being burnt.
1995 525iA 250k mi
2003 350Z 88k mi
2014 F-150 4k mi