What are two useful things moons provide for the planets they orbit?

Answer 1

On Earth, the Moon provides nighttime illumination, and is the major factor in ocean tides.

On other planets, neither of these is true. With multiple moons, orbital distances and timing can vary, and the moons on Mars are too far from the planet (and the Sun) to provide much illumination. Illumination is irrelevant on the gas giants.

Although tidal forces are present on other planets, liquid water is not. So the one purpose moons fulfill is on Saturn, where they shepherd some of the rings.

Answer 2

Moons can provide gravitational stability by helping to maintain a planet's axial tilt and orbit. They also play a role in generating tides on the planet, which can affect ocean currents and ecosystems.