These devices help develop timing, footwork, and racquet-ball contact, and they provide a fairly intense workout. To keep the ball coming back, you have to hit it harder and higher than would keep the ball in play on a real court, which is a drawback for most players, but can also be useful for players who need to develop more power.
A backboard is better practice, but you can't beat the convenience of a Bungee Tennis or Tennis Trainer device, which will set up just about anywhere. When you choose a direction in which to hit, be aware that the elastic will break now and then, letting the ball fly.

