Suppose it's difficult to lift your arm to the ceiling without some sort of nagging pain. Maybe you're recovering from an injury or it just developed over time.
Under the cuff that your deltoids form to protect the humerus in the socket and also under the scapula are 4 muscles known officially as rotator cuff muscles (supraspinatus, infraspinatus, teres minor and subscapularis).
They're small but so important because arm lifting and turning won't happen efficiently without their fitness, the body is a team after all.
This exercise might help these 4 rotator muscles gain the strength needed to stop nagging (they just want TLC). They're small muscles responsible for rotation of the arm, so the movement is small but powerful enough to feel work going on. Let pain be your guide, stopping just before you encounter any.