The strategy was obvious: hire a guy that had already established himself as a world class belter with another band and voila, instant success. Ronnie James Dio was a veteran coming fresh off of major success with Rainbow when he was brought in to front Sabbath 2.0. The resulting Heaven and Hell album reinvigorated the band's career (it's still considered one of their best to this day) so it made sense once Dio left to form his own band that founder Tony Iommi would look for another veteran presence to carry on the Sabbath mantle.
Actually, down to just himself and bassist Geezer Butler as far as original members, Iommi was originally going to jettison the Black Sabbath name altogether and start a new project... hey, it worked for former Deep Purple guitarist Ritchie Blackmore with Rainbow, and was working for Ronnie James with his new eponymous Dio outfit, so why not? Ultimately, cooler heads (ie. management) prevailed, and another DP alumni - Ian Gillan - was brought in to provide vocals.
Most of Born Again is firmly within the Sabbath mold (see: "Disturbing the Priest", "Zero the Hero") but the title track is a different beast altogether, moody and emotionally charged, an experimental blues rock number with a bravura vocal performance by Gillan. "Born Again" is basically a power ballad for the cutter set.