Took awhile to find the right search terms "PTO shaft cap"
Re: PTO Shaft Cap
On my TC29D, the mid pto has a plastic push on cover that has never been taken off, and probably never will be.
I have the screw on cap for the rear, and put it on when I'm not using the pto.
The manuel say's<font color=blue> " The PTO cap should always be installed when the PTO is not in use"</font color=blue>
It does not give the why. I figure it's to keep dirt, debris, and moisture off the shaft.