My son's former pitching coach used one all the time. To show kids where they were at, to "climb the ladder", to show difference between pitches. When the gun is always there it loses its mystic.
He put it to it's best use to show kids how over throwing doesn't help. He'd ask them to "climb the ladder" past 100% to so hard you shit your pants. Almost inevitability the speed would be less than 90%, or even 80. Even at 10 yrs old it's hard to argue with the gun.