Because, the moon's gravitational pull.
The side of the earth that's facing the moon will have high tides, due to the gravitatonal pull.
That being said, the moon effects the tides because of the gravitational pull between the earth and moon.
Without the moon we wouldn't have tides, so the moon is very important.
High tides occur every 12 hours and 25 minutes.
It takes approximately six hours and 12.5 minutes for the water at the shore to go from high to low, or from low to high.