Some luthiers have different habits... When a pickup winder gets done winding a pickup sometimes they will loosely wrap a piece of masking tape around it so when someone comes by they know that the pickup is done... and also as was suggested... this keeps dust and other thing that could lodge between the wire or sit on top of the wires and leak out very small amounts of current... giving the pick an undesired tone... very slight but nonetheless. The tap DOES serve a purpose, although minor...