Mobility of shoulder blades (scapula) and correct alignment of scapula are important for the shoulder joint stability, injury prevention and overall healthy shoulders, such as rotator cuff injuries, shoulder impingement, labral injuries. If you do weight lifting, playing sports like tennis, golf...etc., you want to keep your foundation strong- your shoulder stabilizers (external shoulder rotators and midback muscles) to prevent injuries.
On all fours, keep wrist under shoulder. Bring shoulder blades together and down, toward spine, and separate shoulder blades away from each other. Repeat 5-10 times, slowly.