What's the difference when you finish your forehand above your shoulder or closer to your waist?
Finishing above the shoulder encourages a low-to-high swing, which is at the foundation of the modern forehand because it generates topspin. By ending above the shoulder, you ensure a high finish.
Finishing closer to the waist is done when you want to come through the ball more. I've especially noticed the benefit of this as a player who ordinarily spins the ball a lot more than most. It's a great way to flatten out the trajectory of your shot.
Still, you'll notice that even as players advance, and they learn to swing through the ball and across the body more, there's still a low-to-high arc to their swing, just not as exaggerated.