Why should we need any kind of quest for unlocking a class beside getting our first, level 1 tool?

I'll make you an example. Say I want to be a guitar player. I go buy my guitar (my tool) and "equip" it, maybe I even go and take classes from a guitar teacher. Hooray, now I'm a Level 1 guitarist.
  • Am I completely incompetent and inexperienced? Hell yes, I just started out.
  • Can I become a good guitarist if I practice every day? Why, of course, practice and grab exp to level up your "class".
  • Can I become a Rock Star by practicing every day? Hell no. To unlock the "job" for the "guitarist class" you've got to go through a lot of quests (improving your style, making yourself famous, get a contract) that practice alone won't get you. You need to unlock that.