text: It doesn't really have a purpose in the since that a firefighter has a purpose, but there are consequences of its being there.\n\nThe moon formed when the proto-earth was hit by a Mars sized asteroid during the very early years of the solar system - about 4.5 to 5 billion years ago. The resulting collision knocked a bunch of stuff into orbit around the Earth. That stuff quickly accreted together and the moon was formed.\n\nBecause the Earth has a (relatively speaking) large satellite there are a couple of consequences. Tides are the obvious one. But more important is that the gravitational attraction between the Earth and the Moon provide a weak link, but enough of a link that the Moon stabilizes the Earth's rotation. Right now our axis of rotation is about 23.5 degress tilted with respect to our orbital plane. It has probably always been close to this. \n\nWithout a large Moon, the small gravitational tugs from the other planets can lead to an instability where the axis of rotation (relatively) suddenly shifts - maybe from 23 degress to 85 degrees. This sort of jump has probably happened on Mars.\n\nIt is quite possible - though this is speculative - that because the Earth has never had its axis shift a great deal that the climate has always been stable enough for animals and plants. \n\nIn other words, without the Moon, it is quite possible that only algae and other primitive creatures would exist on Earth.