Tool’s guitarist Adam Jones is a long-time fan of drop D tuning. This allows him to get weight into his phrases while establishing a tight, identifiable tonality that sits alongside the bass.
Many guitarists have favored drop D to add thickness to their riffs. The list would include Eddie Van Halen, Jimmy Page, Kurt Cobain, Kim Thayil, Jerry Cantrell, Dimebag Darrell, Tom Morello, Adam ...