Again, using the first video I posted. That is a clean F-16D block 50. Why does hew need trim? A number or reason. That specific aircraft normally need ruder trim, the way the rudder Integrated Servo Actuator (ISA) is rig, a quirk of that specific Digital Flight Control Computer (DFLCC), the way the airframe behaves, etc. There is no specific answer. All the F-16 I ever work on, the only trim that was always center (when I inspected the cockpit) was pitch, and that is because at 60knots wheel spin, the pitch trim is suppose to center automatically.