Why do many aircraft have both all-moving tailplanes and elevators?
Moving each part has different effect on drag:

Moving just the elevator increases camber, producing large force which is useful for manoeuvring, but it also produces more drag.

Moving the whole stabilizer, keeping the camber low, produces less drag for the same force, which is good for cruise efficiency.


That's why the elevator is used for control and the forward part of the stabilizer for trim.

The arrangement adds complexity, but it also adds redundancy. If one of the actuator breaks and jams, the aircraft is still controllable using the other part enough to make a safe landing.