The moon gives us light at night, but the most important function is the affect of gravity it has on the oceans. The moon causes the tides in our oceans, which helps to keep the water moving. The currents (movement) of the water helps to circulate the warm and cold temperatures, spread the food around (many small marine animals simply float in the tides, so without the currents they would stay in one place), and are responsible for some weather patterns (storms moving in from the Pacific cause rain to fall inland, hurricanes from the Atlantic also move large amounts of water around the planet). Basically, without the moon, the ecosystem in the ocean would be very different: different animals, plants, and mammals would exist. It would also affect life on the land. Plus, the moon is really cool to see when it is full and near the horizon!!