Quick answer
A solar DC cable spends 25 years or more outdoors, and its fine class-5 copper strands would slowly oxidise. A thin tin coating on each strand blocks that corrosion, keeps terminations reliable, and preserves the conductor over the cable's life. The trade-off is honest and tiny: tinned conductor resistance sits about 2% above the bare-copper maximum — which is why a genuine solar cable datasheet shows, for example, 8.21 Ω/km at 2.5 sq mm instead of the bare-copper 7.98. See APAR solar cables.