Govenors
Cities can (and must) be governed (think of the total war series of video games). Governors will have traits that act like multipliers for local food, health, administration, gold, culture, research or military production. A bad governor can ruin a city. A good governor can turn a backwater hole into a competitive township and a competitive township into the gleaming gem of your empire.
You have to govern your cities. But you can also have only so many leaders. And they may be governing other cities or commanding armies. If you can’t find a leader to govern your leader you must use your faction’s default leaders (which generally aren’t as good as specific leaders you develop and grow throughout the game and which usually also require a salary).
Removing a non-default leader from your city takes an entire turn during which time the city is treated as having a default leader. Only during the following turn can the leader be applied elsewhere.