different reasons for different aircraft.
Concerning the F-4...
The original design was unstable at high Mach numbers and did not have acceptable low speed flying characteristics for carrier operations. McDonnell overcame the high Mach number problem by incorporating 23 degrees of anhedral for the horizontal tail and 12 degrees of dihedral in the outer wing panels, yielding greater stability at high mach numbers
Concerning the Harrier...
The anhedral angles of the wing and horizontal tail are intended to minimize the aircraft rolling moments due to sideslip. Even so, at certain combinations of low speed and high angle of attack, aerodynamic rolling moments greater than the combined aerodynamic and reaction control power may occur if the angle of sideslip is allowed to exceed a prescribed value. To assist the pilot in maintaining the angle of sideslip within acceptable limits, a small yaw vane that provides a visual indication of sideslip angle is mounted on the fuselage just ahead of the windshield.
Concerning the Mitsubishi T-2 jet trainer...
The tail assembly is conventional, featuring slab all-moving tailplanes with an anhedral of 15 degrees. The steep anhedral kept the tailplanes out of the engine exhaust while allowing them to remain effective through the wing wash.