It is the equivalent of a PCV valve.
Normal the intake receives a known quantity of air (and crankcase vapors) through this metered oriface. The purpose of all PCV (positive crankcase ventilation) systems is to give the blow-by gases of combustion somewhere to go other than the atmosphere. Old cars before 1968 just had a breather; usually where the oil was put in at.
Because the amount of crankcase vapors varies with speed and engine condition, the amount of metered PCV was established at a much greater volume than anticipated volume. In order that a vacuum would not be created in conditions where the meterd amount was more than the actual produced amount, there is the other hose going from the valve cover to the aircleaner. Airflow in this hose goes from the aircleaner to the crankcase and makes up the difference.
The exception is on worn out motors. In these cases the metered amount is less than the produced amount (exessive blow-by) and airflow is from the valve cover into the aircleaner. It is not a good sign to see oil in the aircleaner at this hose.
Bosch Master, ASE Master, L1
33 years MB technician