If the parts of the gasket (circled in red) don't seal, then the bolt threads are exposed to the pressure/vacuum in the valve cover depending on the throttle percentage. That's one pathway to leak air/oil in and out of the intake system because the bolt isn't sealed in the valve cover. A crack in the valve cover at that point (can occur from reusing gaskets or overtightening) would also cause a leak. I had that happen on a motorcycle gearbox cover.


Valve cover gasket

As an aside, there's a company in Australia manufacturing TZ750 crankcases and they perform a process called vacuum impregnation on their cases to "seal up" any porosity in the finished article. It's an interesting process.