Why did God make the moon?

The moon has several purposes: It provides light(through reflection of the sun's light), its gravitational pull causes the ocean tides, which ensure a steady mixture of oxygen from the air into the water, keeping the ocean life alive. More importantly, up until less than 1500 years ago it was how the world kept track of the calendar, by counting the full moons. That's what allowed them to keep track of farming seasons and stuff like that.