Activities that you need it for: You really only need this muscle to raise your arm up, it has one job and its designed to do that only that.
Injuries associated with it: Adhesion, shoulder impingement, rotator cuff problems (strains and tears, shoulder bursitis. When a rotator cuff actually tears this is usually the muscle that tears due to excessive load on it.
Our take on the muscle: This muscle gets beat up way too much in any shoulder problems and often is the one that the surgeons like to go after because they don't fully understand the biomechanics of the shoulder and assume if the muscle tore then it must be the main problem. That logic and thinking is why a lot of people end up doing really poorly with shoulder surgery and repair! The tear itself is usually caused by other things not working correctly, more specifically the other muscles (infraspinatus, subscapularis and teres minor.) The force has to go somewhere and usually it ends up going into the supraspinatus muscle, which tears rather easily because it really is only designed for one job, not a bunch of other ones when the shoulder isn’t functioning correctly. So the take home here is simple: Don’t rush into surgery for your shoulder, get it evaluated by a biomechanics specialist and see what all can be done conservatively.
Shoulder injuries are managed very poorly in our current medical model and jumping into a surgery that you might not even need can actually prove to be a bigger problem down the road.