True what wiging19 said. It does let you focus on the car a bit more. For example if you look at EVIL72's car in his sig, then imagine it with black walls, it does give more focus on the car itself. Sorta more modern and streamline looking.
But IMO I always liked the raised white letters on 14"/15" rims (E-bodies anyway). It looks more period correct IMO. The white letters spinning around when you're moving look cool. 17" and up rims look better in blackwall. I like redlines on the pre '71 B-bodies with stock rims.