This is theory!!!
Its type dependent Genhis and its written into some aircrafts maintence scheduals.
Basically its to test that the selector valve is working so it does actually shut the fuel off from going through the fire wall. I think its for only some valve types depending if the off position can fail and still allow fuel through.
I think some of them the selector operates a left and right flow or both and when you turn it off its a seperate off valve that closes or you have a three port left, right and blank shut off.
BUt it could be of course to do with your good point that a none gravity feed engine will stop sucking so it won't keep fuel going through the fire wall if you kill the engine.
Its only a theory its been years since I did one for a SEP and i can remeber checking the valve but can't remember if we timed it.