what applications are used for black rubber electrical tape?

In electrical maintenance we use rubber tape as the first layer of insulation over split bolt splices. First a layer of rubber tape stretched tightly over the connection, the a layer of linen tape to cover that(some use friction tape) then either vinyl electrical tape or liquid electrical tape for water resistance. We called this bundle a "dog knot" and it worked quite well on 480 volt 3 phase lines throughout our plant when we had to tap into them to add machinery or make repairs.

for electrical work in damp or wet areas (example) splices from the electrical company to your home, street lighting, parking lot lighting.

isolation of bare electrical wire, binding electrical wire to make a bundle. holding or placing a bundle of wire (could be computer wire) at a specific location. sealing a "join" or splice of wire. I would say that isolation is it primary function.