I had a plumber answer this question today but I thought I would post it here to see if I was the only inspector that didn’t know.
Each photo is taken in the same house currently under construction. Why is there a screw in each drain?
Damn if I know Gary…water level for gravity test?
I don’t know either, but will guess is/was to hold the test ball so it don’t fall in if deflated.
Marcel 
Keep pop cans and dirt out also to stop some one from Peeing in them .Screw is to keep the cap on so they can not lift it off.
… Cookie… What do I win ?
Roy, is that a comment from someone’s experience in doing that? :mrgreen:
You would not have done that yourself would you? ha. ha.
Marcel
You get to be the first one to pee in them.
No way my two best buddies on the job where Brickies and Plumbers .
Two way street we helped each other all the time .
…Cookie …
Of course what else would you expect me to say to a brickie
Ok, we will say that the roofer did it. ha. ha.
Marcel
Dale…no wonder you are the USA member of the year. They install one drain with a long header to hold a certain amount of water pressure for a hydro test. They put the screw through the remaining drains so they can break the vapor lock that can occur prior to proper venting being installed. Remove the screw and the drain vents.
Anyway…that is what the plumber told me.