No reason that I'm aware of. If the PCV fits tightly into the grommet and the grommet provides a complete seal around the PCV, that's what you need.
The PCV (Positive Crankcase Ventilation) system allows crankcase fumes to be drawn out of the engine, fed into the intake, and burned during engine combustion. Up until the early 1960s, many cars just vented those fumes to the atmosphere. Here is a generic description of how the system works with a typical small-block Ford V8 engine. The inlet to the PCV system is through a plastic fitting on the passenger side of the air cleaner housing which has a little filter element inside the housing (known as a "breather element") that keeps debris from being sucked into the crankcase. There is a hose from that air cleaner fitting that runs over to a plastic fitting in a rubber grommet on the passenger side rocker cover. Fresh air is drawn into the crankcase through this hose. The outlet is through the PCV valve on the driver side of the engine. The hose from the PCV valve connects to either the base of the carburetor, a spacer plate between the carb and intake manifold, or a fitting in the intake manifold.
