What is the advantage of using tinned copper as conductor in cables?

Basically, tinned copper conductor is used to avoid oxidation. As Copper react with Air, it gets oxidized and turns green. It creates kind of semiconductor layer on the conductor surface which results in poor conductivity at terminals where cables insulation is removed to terminate them.

Tin coating helps in protecting copper conductor against such oxidation. It also prolong the expected age of the copper conductor by mitigating the corrosion chances which causes deterioration.

For further details, you can refer to Electrical Material Books. Regards,