Here is a great way to stretch that posterior shoulder cuff/capsule that often gets tight, particularly in the dominant arm of overhead athletes. This tightness leads to limitations in shoulder internal rotation and horizontal adduction.
With cross body stretching, people will often allow the shoulder to dump forward, defeating the intent of stretching the posterior shoulder region. This exercise is great because it will allow the head of the humerus to stay posterior, which will further the stretch. You are essentially giving yourself a posterior glide of the gleno-humeral joint.
In addition to the posterior glide, this stretch will also address internal rotation and horizontal adduction limitations.
Note: Taking up the slack of these muscles initially through horizontal adduction is often more comfortable than internal rotation.
Citation: Posterior shoulder tightness and rotator cuff strength assessments in painful shoulders of amateur tennis players. 2013. By: Marcondes et al.