Question

why the plastic covering of electrical wires must be removed from the ends before making electrical connections?

Answer 1

Because the plastic covering are insulators i.e.they don't allow electricity to pass through them hence we have to remove it from the ends so that electricity flows through the wire

Answer 2

The plastic covers are insulators. Insulators are the materials that does not pass the electric current through them. So the plastics do not allow electric current to pass through them. So plastics must be removed from the ends.