Yeah they never explain this. It's even more confusing that Black Mages uses fire against bosses made of literal fire to kill them.

I do miss the elemental strategies of the offline Final Fantasy games. I realize that the game relies heavily on rotations that need to be mastered and making elemental affinity would cause the game to become a lot more stressful.

Still, I think something along the lines of elemental levels would be a lot of fun to customize your character without affecting the rotations.