Optimal Alignment
Dancers require the ability to maintain their pelvis in a neutral alignment during most movements in dance. This is actually the optimal alignment for the average person as well! Most people nowadays have a tendency to allow their pelvis to tip forwards and which increases the curvature of their low back.
People with chronic low back pain often have tightness in their hip flexors (the muscles in the front of the hips) and have weak abdominals. The forward tilt of the pelvis creates a pinching in the joints of the spine and, over time, the pinching can lead to small fractures and, if left untreated, slippage of one or more of the bones in the spine. Shifts out of ideal alignment create imbalances in the muscles and can cause narrowing of the pathways that nerves are supposed to go through, which can create sciatica symptoms.
Dancers and gymnasts frequently try to place their bodies in more flexible positions such as leg extensions and backbends. Achieving high difficulty skills or more impressive leg extensions requires the spine to start from the neutral alignment. Side leg extension in particular require the dancer to maintain an upright pelvis to prevent the front of the hip socket from blocking access to better extension. Most acrobatic skills are safer and correctly performed from a neutral rather than turned out leg position which is difficulty to attain with tight hip flexors.