Some theories conjecture that our moon was created from a large asteroid colliding with the Earth and that is composed of the lighter parts of the Earth's outer mantle. Since its creation, it is obvious by the impact craters that the moon has continued to run interferance (a sweeper?) for a lot of other nasty big rocks that may have otherwise impacted the Earth.
Also, it has saved millions of drunks by lighting their way home from the pub who might otherwise have tumbled into a ditch and drowned.
It also makes tides and gets young women in the mood for lovemaking.
Lets just leave it up there.