The shoulder blade (scapular) is the base of your shoulder and arm movements.
Poor stability results in downward tipping and rotation of the scapular. This causes your acromion (bone) to pinch down into the subacromial structures (eg bursa and tendons) causing impingement leading to further injury. Poor scapulo-humeral rhythm is a significant cause of rotator cuff impingement which must be addressed in rehabilitation.