What's the purpose of the backswing?
To create power.
The single most vital purpose of the back swing is to create a series of reflex movements away from the golf ball to create maximum power on delivery of the club head on its return back to the golf ball through the point of impact.
Finding the most powerful path for delivery is a matter of creating a position at address that will allow all the body parts to blend and respond equally. In essence we are looking for total connection at address in a search for total connection at impact.