If you struggle with shoulder pain and a stiff back, this movement is going to quickly become your best friend. It'll clean up those areas efficiently with one single movement.
The coordinated movements in this drill increase the mobility of the shoulder and thoracic spine. It'll also help you move smoothly to reduce pain and increase functionality.
This is more than just stretching passively and hanging out on your joints, which has become all too common in the self-help rehab industry. If you can't out-roll or out-stretch your shoulder pain, you need to be more active. This drill is your best bet.
Use this movement in the dynamic warm-up before training with 8-12 reps per side, moving slowly through the range of motion. Do it on off days too to get yourself out of that nasty forward-flexed position and remediate your posture.