Posture is the window to the spine.
If posture is out of balance, then the spine is out of balance, putting unhealthy pressure upon your nervous system. Activities as subtle as sleeping, standing or walking abnormally: to more obvious traumas like emotional stress, work or auto injuries, recreational injuries and even the birth process itself can result in spinal imbalance.
Your spine is the most important factor affecting your posture. In order to have the best possible posture (and health), your spine has to be in the best possible condition.