It's interesting how when we think of great singers, we immediately think of someone who sings in a high register. I can tell Ken Tamplin has a strong background in hard rock music, and his program reflects that by training people to sing high (which is the reason I sought him out). I too aim to be a hard rock/heavy metal singer.
But what about styles that have very low vocal registers? What if someone's goal is to sing very low, like Johnny Cash? Personally, I don't have a use for it myself, but I was just curious to know if that type of thing would be worth learning about.