The Expedition Leader and its replacement the Mayor let you assign other nobles.
The baron and upgrades, along with the monarch, basically serve as a good source of fun mandates and demands to give you goals -- though they should allow a wider range of nobles than the mayor, this is essentially bugged
The Sheriff and its upgrade, Captain of the Guard, lead your Justice squad. Unless you're working on caring for mandate-y nobles, this is really more harmful than good.
The Manager lets you profile workshops and order tasks
The Bookkeeper counts every item on the entire map and tells you what you have in your stocks
The Broker appraises items and trades for you way better than random nobodies.
The Chief Medical Dwarf allows dwarven medicine to take place
Militia and Squad Leaders are the bosses of your military.
As you can see, most of the folks that appear on the nobles screen are actually moderately hard working members of your fortress, enabling critical game systems to come online... it's only really the "noble" nobles (Baron, Count, Duke, Monarch, their Consorts) that do nothing but make your life a pain. And, in this edition, they don't tend to be replaced when they meet with unfortunate accidents. In older editions there were more posts, the mountainhomes would send new magnates, and there probably will be more posts and immigrant nobles again in the future (like the Hammerer for upgraded Justice and the Dungeon Master to let you tame exotic beasties) and more headaches coming from your ruling class. For now things are actually pretty tame compared to the tantruming psychopaths of Boatmurdered or the justice gestapo of Syrupleaf -- nobles that are supposed to be lazy even work when they're told!