I've been playing tennis for a few years now and I have never seen anyone on the court with a tennis ball machine on the other end. I don't understand this, it is such a great tool to practice your stroke, return stroke and every other kind of shot out there. The absolutely best part about having a ball machine is that you don't have to apologize when you miss or run around trying to find a ball, when you should be practicing.
So besides saving you time and energy, what else does it offer you. Repetition. The same shot will come to you over and over and over again. Repetition. This is a key factor in tennis. You will see pros after they make a bad shot will practice the shot they just played, even if it's just in their head.
By the time the real shot will come for you, it will be so apart of you that you will naturally put the right amount of top spin, backspin or whatever shot you need to play to win the point.
Your shots need to become natural like driving a car, this is where I believe the tennis ball machine comes in very handy.