Yah, gasses from the valve covers need to be evacuated so that there is no positive pressure built up. There will be some oil suspended in those gasses. Before the 1960's the gasses could just be vented to the atmosphere & you can still get little filters to allow that to happen. Car manufacturers are however required by law to supply the vehicle with a means of combusting those gasses rather than venting them. Drip canisters are legal and do work but the manufacturers choose not to use them for cost & maintenance reasons.