Why would a shop owner used a marked dime to start up his jukebox, and then later take that particular dime back from the coin box? Why does it matter which dime he took back?

While I'm at it--on a different point that has confused me since I was a kid: Superman's key to the Fortress of Solitude being disguised as an airplane direction marker. Are there or were there ever such things as airplane direction markers in the arctic?