This is an exercise I like for shoulder mobility and strength. It affords us the ability to get a big stretch across the back of the shoulder and upper back, and put the biggest load on the those muscles in this stretched position. This joint angle and muscle length is similar to where we get maximum power from the lead shoulder in the swing. This is best developed by swinging at maximum effort. This exercise is simply one option for strengthening the shoulder muscles at a long muscle length, which is desirable.
As you get comfortable with it, you can ramp up the weight and explosive intent.