In order to answer this question, let's gain a simple understanding of how the body is designed to move. Natural Movement for the spine is to fully flex (picture above on the right) and extend (picture above on the left) one vertebrae at a time. This occurs when our bones are in the right position and controlled sequentially by the 'right' muscles. The best template for optimal sitting posture is a toddler, who sits in a natural extended position at the end of a chair,
without having to consciously think about it.
Whether it's a function of sitting too much in the wrong position, compensating due to injury, or daily patterns creating imbalances, the spine loses the ability to naturally flex and extend, and we might develop a disconnect (tension, lock) somewhere along the spine. When this happens, we cannot fully flex and extend the spine to
sit up straight (picture below on the left).
In order to see ahead, our eyes have to be level - if we trace the model on the left below, her eyes would be tilted down if her head simply followed the angle of her upper back; so she tilts her head up. Well, that's not going to happened, so she begins to overuse the wrong muscles to look up or maintain the sitting position. This moves her bones out of naturally alignment, which can create a downward cycle of pain, strain, and injury.