It's there so you can check the gap between the clutch disk and the flywheel. I am sure someone can post up the required clearance with the clutch fully engaged.
Is it .040" (minimum) and .060" desired if you can get it?
Rocky
PS> I don't believe the hole is to let hot air out - it's too small. I do believe the cover might keep things from falling in there.
I put timing marks on my flywheel, and a pointer in there, and I can adjust the timing from the back of the car. See my 5357 thread.