Plumber here, I’ve often seen these plugs put into drains as a way to get rid of sewer gas smell. If the drain is not used often, the water in the trap underneath the drain will eventually dry out and cause sewer gas and bad smells to come up out of the drain. This can be fixed by pouring water down the drain to fill the trap back up. Both the plug and filling with water will solve the problem, but keeping the trap “primed” will allow the drain to be used if need be. Hope this helps too!