Bones that are out of line are at the mercy of gravity and put strain on supporting muscles and ligaments. You can cause the same kind of stress if you slouch with your shoulders too far forward or if you stand with more weight on one leg than on the other. Tension headaches, muscle soreness and back spasms can result. Chronic poor posture can lead to degenerative changes in the spine itself. It won't be able to take daily wear and tear if the parts are always out of kilter.
Some people think of good posture as being rigid and stiff.
In fact, it's just the opposite. Good posture is comfortable and natural-looking because it's the normal position. Hold your head straight, balanced evenly between your shoulders and with your chin parallel to the floor. Keep shoulders, hips and knees level and your feet and knees pointed straight ahead. From the side, your ear, shoulder, hip, knee and ankle should be centered along an imaginary straight line.