According to Prof Reid :

Is there some quality, durability or other reason for the wipe?
Or, is it just cosmetic, and essentially unnecessary?

Mostly cosmetic, however...

Plumbing flux is corrosive to copper. Generally speaking it won't
cause a problem (because the quantities are small), but it does
leave you with greenish stains.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.