Yes, I use them, as most have described above. I use a dog Chucker to teach hitting and to throw the tennis balls to the outfield. Most tennis clubs sell a large sack full for $5.00.
The Chucker drill came from Right View Pro. I learned it a UofAZ's coaches camp.
Warrior, It is so simple. The student stands in the box, puts a tennis ball in the chucker - and swings it, exactly like her swing. If you teach hitting, you will know what to do.
We face the backstop, so we don't have to chase so many tennis balls.