to make it hard for competitors to take them apart and reverse engineer them
-thats the main reason.
Also, its to stop buzzing of the wires............and to stop moisture getting in there and settling in there......and to give extra insulation proofing.
Varnish prevents oxidation due to salty atmosphere or moisture effects. Varnish is not a hinderance to any duplicators as it is easily stripped with high quality thinner.
Cheers
Varnish is actually used to avid the windings from Moisture and Oxidation to the wires and also to avoid flash overs increasing the insulation.
It can not stop the duplicators as some times they are more Genius :-D.