The traditional band pull-apart is best programmed as a warm-up or primer movement for anterior-chain dominant training (lifting with the front side) like right before you bench press. When programmed with multiple sets between 10-20 reps, this movement provides the correction and activation the posterior shoulder girdle needs to function pain-free and support big pressing movements.
What makes the band pull-apart so effective? It retrains shoulders to function around pain by getting them to move in the opposite direction that they're in most often.
As we sit and slouch, our shoulders stay in a rounded forward posture. The shoulder blade is forced into protraction (forward), upward rotation (up and around the side of the thoracic cage), and elevation in a forward and internally rotated position secondary to rounding at the thoracic spine.
This position becomes habitual. The intrinsic scapular stabilizers and posterior shoulder musculature change as a result. How? Well, over time these muscles become functionally shortened and weakened, which is bad news for the stability and strength of the shoulder joint, especially when it comes to the big compound exercises.
Think of the pull-apart as a practice that'll cancel out shitty posture. When you do it, you're moving the shoulders into horizontal abduction – a commonly neglected plane of motion among lifters.
With the shoulder and scapula moving pristinely, the thoracic spine will also be trained to move into a more neutral position on the backside of the movement. This completes the triad of pull-apart awesomeness.