Why it is advise to clean the ends of connecting wires before connecting them?

To avoid poor contact and consequently the increase of the electrical resistance at the junction.

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.