The Moon keeps the Earth from wobbling like crazy. It keeps the North Pole pointed North (except for a slight 26,000 year wobble). Without the Moon, the Earth would sometimes flop clear over on its side so that its axis was parallel to the plane of the Earth's orbit, instead of perpendicular to it. This is one of the problems trying to terraform Mars, which has no large moon to stabilize its rotation.
I'm no astronomer, but I did stay at the Holiday Inn Express one night, where I saw this particular explanation on the Science Channel!