I saw the Sabbath/BOC show in 1980 (Black and Blue, for those that remember it) at the Boston Garden and I wasn't aware until a few days before the show that Ozzy had been given the boot from Sabbath. Dio tore things up for Sabbath that evening for sure.
Sabbath was the loudest band I'd ever heard to that point. That honor lasted for about 40 minutes until Cult took the stage and made Sabbath sound like a string quartet. Not better, per se, just a fuck of a lot louder.
In any case, Dio was an important element in the whole hard rock/metal thing.