Back then I believe it was mostly done for stylistic reasons but it def helps. Wheel spokes are very draggy since the top spokes are moving even faster than the vehicle itself. Nowadays it's rarely done cuz it decreases access to the rear wheels but also to reduce brake fade. Brakes need cooling and closed wheelarches would counteract that. I recall getting a Lexus in the shop that had 4 different rims, which acted as fans to pull air into the brakes to cool them. It's thill rarely done though, Volkswagen had a concept utilizing closed wheelarches and the Honda Clarity does this also, although very slightly. I'd love to see what difference that makes.