Why would anyone place a flat drain stopper over a shower drain?
Because they can?
To stop the water.
There was probably a lip or something at one point to make it easier to remove. Seems it has broken, deteriorated, whatever.
edit just looked again, that’s JUST a shower, LOL, my bad.
Poor man’s sauna.
To keep the smell away and control the gurgling sound from an improperly vented sewer line.
And there you go again with logic. :mrgreen:
Christopher, wins!!! Take the cover off and the SMELL will make you gag.
In AZ it is to keep the roaches from coming through the drain.
A home inspector was there before you, testing the shower pan, and forgot it?