In nature, the shape of a circle, or sphere, is the strongest three dimensional figure; geometry also posits that the circle is the strongest structural shape. This is true because when stress is distributed equally along an arc, it cannot be concentrated on any one point.
The human body is an incredible structure. It has incorporated this strongest of shapes in the neck, which was designed to curve in order to provide greater shock absorption and weight bearing ability in order to hold up the head. When in a normal, healthy position, this cervical lordosis (neck curvature) allows the spinal cord to remain soft for a better flow of blood and nerves to the rest of the body.