Final answer:
Rotating the patient's shoulders anteriorly during a PA chest projection is done to protract the scapulae, moving them out of the way for a clear view of the lungs and stabilizing the shoulder joint for improved image quality.
Step-by-step explanation:
The purpose of rotating the patient's shoulders anteriorly for the PA projection of the chest is to protract the scapulae. This movement is crucial because it helps to move the shoulders out of the way of the chest, providing a clearer view of the lung fields during a radiographic exam. By positioning the scapulae in this way, the anterior structures of the pectoral girdle such as the clavicles and the serratus anterior muscles help to stabilize the shoulder joint, ensuring better image quality and diagnostic accuracy.