Natural satellites are usually orbiting around a planet. Just like the Earth's Moon, satellites play important roles to the main-body planet. A satellite preserves much of the angular momentum of the planet-satellite system, stabilizing it, provides a measure of time with its periodic motion, and has a tidal effect on the planet.
According to the Unification Thought, the central planet and its satellite are reciprocal subject and object partners. They are in either circular or elliptic orbit at various inclinations in general. The orbital characteristics of a satellite give us information on the mass of the central planet and the origin of the planet-satellite system. Satellites around Jupiter and Saturn could be called mini-solar systems, giving clues on the form and origin of the entire solar system.
Other than the very practical reasons of God's creation of the Moon, natural satellites must have been created mainly to show humanity the variety of satellite systems for providing the above-mentioned information, as well as making us realize how special the Moon is.