B U L L E T P R O O F the stability in your shoulders to get stronger or before you injure them!
The shoulder joint has the biggest range of motion out of all the major joints in the body.
As you can see in the photo the contact between the arm (humerus) & shoulder joint (AC) is quite minimal. 
The analogy used is it’s like a golf ball sitting on a golf tee.
Only having 30% of the humerus surface in contact with the shoulder joint.
This is great for being able to move our arms around wherever is needed, it does cause problems when lifting weights or playing dynamic sports though.