In the golf swing, your shoulders need to go through full range of motion so that you can create a wide swing arc and clubhead speed. It is important to do shoulder exercises that take the shoulders through full range of motion and in different planes of motions.
Stand with feet shoulder width apart and arms bent at 90 degree angles, fingers pointing up in the air.
Slowly rotate shoulders so forearms drop parallel to the ground, palms facing the ground.
Return to start position and repeat motion.
Do 8 to 10 repetitions.
This can be done with or without weight. This will strengthen your rotator cuffs so you can generate more clubhead speed.