Shadow-practice is when you practice a stroke without the ball. It's the best way to develop proper technique and streamline your strokes, especially with help from a coach or experienced player. You do need to combine it with practice at the table, with a ball, so that you can develop the stroke with proper timing and racket angle. But trying to do all of this at once is difficult, and shadow-practice allows you to zero in on just getting the technique right.
So . . . when should you shadow-practice?
Suppose one of your strokes doesn't feel right. Do you think you have a better chance of getting it right by practicing it while also trying to hit a moving ball, or by shadow-practicing it until you make a habit of doing it right, and then doing it with a moving ball?
Suppose you miss a shot because you didn't do a stroke properly. Do you think you have a better chance of getting the stroke right the next time, perhaps in the next rally, by not practicing it until then, or by immediately shadow-practicing the stroke as it should have been done?
Suppose you miss a shot because you misread the incoming ball's spin, speed, depth, or height. Do you think you have a better chance of getting it right the next time by doing nothing, or by immediately shadow-practicing the shot as you should have done it, so your subconscious can better connect what to do with any given shot?
Do you think you have a better chance of improving your strokes by only practicing them when also hitting a moving ball, or by regularly shadow-practicing them to develop the proper technique?
If you watch the top players, especially during their developing years, you'll notice that most regularly shadow-practice their shots. When they miss a shot, many will shadow-practice the shot as they should have done it. There's a direct correlation between those who shadow-practice to get their shots right and quickly improve, and those that don't and don't improve nearly as quickly.
So . . . why not make shadow-practice a part of your practice routine?