A pro once explained it to me in a lesson, the body turn and weight transfer adds power to the shot if synched correctly - almost like torque would if we were discussing motors or vehicles etc.Everyone has to make their own judgement but I know for myself that when I get my body turn and arms in sync I hit it better, truer and further than arms alone. Arms alone are a disaster for me. Each to their own though.
I know when I got it right, I could hit the ball miles (driver + 5 iron through a 490 yard par 5)
Sadly those days are a distant memory. ?