This is a common question from younger folks who dont remember the "road draft tube" engines before the Positive Crankcase Venelation arrived. There is an amount of vapor created in the crankcase.....older engines with worn rings and cylinder walls have more blowby. In the old days, the oily vapor was simply routed out of the engine by a tube. This road draft tube usually hung down under the car so the vapors would get blown away as the car ran down the road. The tube had some mesh in it so that the oil would condense and maybe go back into the engine. Those cars didnt rust underneath since they had a nice coating of oil on their underside.

Your engine without PCV is the same.....no matter how tight it is there will be vapor escaping and the valve covers will have a nice oily film on them......if you dont care, the system will work.

However, it is so easy to install a PCV, a clean engine compartment is simple.