Hi CD, well one theory states that the moon was formed when a Mars sized planetoid struck Earth a glancing blow while strill very young. the resulting collision sent material into space forming a ring system such as Saturn has today, but unlike Saturns rings our planetary ring formed the moon.

As a result of the collision between  two planets Earth gained mass, and formed a metalic core, this was to become vital for future life on earth, because this metalic core and the Earths axial rotation gives us our "shield" (Van Allen Belts) against the sun, and allowed the Earth to retain its atmosphere. The Moon is also vital, as it helps to stabilize Earth from wobbling to much, which unchecked would cause extreame seasonal shifts causing havoc with any life trying to gain a foothoold.