If dirt and grease were good conductors of electrical current, then we could make wire
out of dirt and grease instead of expensive copper. Sadly, they're not. So a coating of
dirt and grease on the wire can have a substantial impact on the connection, and can
even block the flow of current across the connection completely. Moreover, in the case
where the ends of the wires are to be soldered, solder does not adhere to dirty wire.