And even if you say the classes are descriptive, you have to design them that way.Very true. Some classes are prescriptive (I am a wizard, that is my job) and some are descriptive (I am a treasure acquisition expert, but my class is called rogue by others). And some straddle the line. (I channel divine energy the same as any priest in the seminary, but I'm not a cleric in the sense that they are).
If you are of the opinion that all classes are all prescriptive (that is, fighter is a specific profession) then you need something to represent adventurers that didn't go to fighter/wizard/cleric/rogue etc school. But if you argue some or all classes can be descriptive, the need is much less.
3e, 4e, and 5e were designed heavily into single concepts. You couldn't squint and play anything. This is why 3e and 4e bloated with classes and 5e with subclasses. That's why the barbarian and sorcerer even exist.