So, before Shadowbringers came out, I remember there being some interviews and such where Yoshi-P dropped hints that with GNB being added to the game as our 4th tank, we'd likely see a split in the tanks, where two would be more MT focused, and 2 would be more OT focused. This, however, did not happen, and all 4 tanks are now capable as MT or as OT equally, with the playerbase being the ones who have made decisions on which tank suits which role better (EG GNB prefers OT because moving a boss inside Continuation windows was not a fun time)
However, we look at healers, and the same dichotomy got applied, two 'main healers' (or Pure), and two 'off healers' (or Barrier). You can even divide them up in a 2x2 table like so:
As we go through the tier though, people get more gear, and we run into the eventual issue that seems to occur every raid tier: the two healers with raid buffs, SCH and AST, scale off of not only their own gear, but the gear of other people due to Chain Stratagem and Divination/Cardspam. This leads to them pulling ahead in RDPS, the metric that everyone cares about. It's all well and good saying 'oh well you see, WHM has a lot of powerful healing tools to save a run when things are going badly' but healing is a very binary thing in this game, you either have enough healing to clear, or you wipe.