The whole FPV hobby, and especially multirotor drones, work only thanks to control theory and PID controllers. They are the only thing that separates chaos and immediate flip-over from a stable flight. A regular multirotor has at least 3 PID controllers. If you enable any mode with self-leveling, the number goes to 6. Any GPS assisted mode increases the number to at least 12.
Of course, the PID controllers we think when we think about when we say about multirotors, are Rate PID controllers. They keep our drones stable by keeping angular velocity at desired levels. And usually, they are the ones that have to be tuned to achieve a nice, smooth and stable flight.