[bore] Years ago, I read in the technology of soldering, by the bloke that started multicore, that old tinned copper wire is difficult to solder, because the surface slowly cold-alloys, and is in some way I no longer remember, fully satisfied so that it will no longer form a surface alloy with the solder. [/bore]
Yes, it's to protect against oxidation. There's no such thing as untinned copper wire in automotive use, for example.
As for soldering, it doesn't really make much difference. Tinned copper wire still needs tinning for soldering, which still requires bringing the temperature up just as high so the flux can remove any oxidation or other residues, and solder can flow. So it really doesn't make any difference, except it takes a lot longer for exposed tinned wire to oxidise noticably.
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