A bit obvious really but...

The earth itself has many resources that are derived from the energy of the sun. These resources don't just simply disappear into thin air when the sun goes down. On earth there are many atmospheric processes that are intimately linked to biological processes and mechanisms that help convert this energy into the many differing forms that can be used to sustain all the many different forms of life found on the planet in a cyclical manner. After the moon itself took on its final solid form from the same mass and gaseous like matter to have come from the sun along with all the other planets in our solar system, the newly formed satellite then began to serve a possibly accidental means of helping to regulate these cycles here on earth. The moons obit influences the movement of the tides through the effects of gravity. I might also point out that the tidal movements also influences the reproductive habits of many aquatic species which are also exhibited in many land creatures too. In essence this might also give some indication as to the origins of many land creatures in  the Darwinian sense that haven't quite yet evolved more complex biological clocks that are dependent on a whole host of other factors. Or it maybe just due the fact that it provides rich pickings for certain land predators.

Incidentally we can only see the moon at night due to the sunlight reflecting from the surface of the moon.