People need to stop thinking that the arm has little to no relevance in pitching, it is the single most important element people!!! The arm is the last step in the linear train before the ball is released, it is the thing that transfers all the energy your body has built up into the ball, and it adds a significant amount of energy into the throw as well. Without a strong, healthy, well conditioned arm that travels with good speed, you are losing potential energy that could go into your throw.
So please stop thinking that the body throws the ball and the arm is along for the ride. The arm throws the baseball, but good fastballs are made with the combination of strong lowerbody, powerful core rotation, and MOST IMPORTANTLY an efficient arm action that transfers as much of that energy as possible into the baseball. The arm is only along for the ride in the same essense that an engine is just “Along for the ride” inside of a car.