I believe that Bob hit it right on the head. I have had both with and without them hooked up and I don't like the
oil fumes on the covers plus the smell. On my 1958 Ford, the factory put a tube that plugged into the back of the intake and ran down around the bell housing and just emptied the fumes to the ground. There was always a mess at the bottom of that tube until I extended it about 1" and then the wind going under the car took it away. Since then all of my vehicles have had hoses to both valve covers, usually with a PCV valve in one cover and a twist on cap with the hose going to the air cleaner in the other cover.
Ron