It's a naturally imaged concept in which you start off in a class, kinda like school you learn the basics of what a tank is with its basic rotation and then you branch off into multiple Jobs when you leave school.
The aggro AOE for example is something that all tank jobs use each with their own aesthetic and names but they all play the same usage. This Basic rotation will be available at the class level and each Starting City will then in turn have a class based on this role (Tank in this case) once you max that class you now have an option to branch out into multiple jobs with their own advanced rotations, style and aesthetic just like in real life.
A gladiator may take a dark path, discarding his/her shield for a darker style the Dark Knight, or may go the other way and continue to embrace the valor of a Shield and Sword Paladin.
One may argue in semantics like the other, saying what does a gladiator have to do with a paladin? a gladiator fights for sport fame and money that's not a paladin, but if we look past these little nibbles, its easier to grasp.