Page "Orbital station-keeping" Paragraph 2

The deviation of Earth's gravity field from that of a homogeneous sphere and gravitational forces from Sun / Moon will in general perturb the orbital plane.

For sun-synchronous orbit the precession of the orbital plane caused by the oblateness of the Earth is a desirable feature that is part of the mission design but the inclination change caused by the gravitational forces of Sun / Moon is undesirable.

For geostationary spacecraft the inclination change caused by the gravitational forces of Sun / Moon must be counteracted to a rather large expense of fuel as the inclination should be kept sufficiently small for the spacecraft to be tracked by a non-steerable antenna.