Improvements may come in variety types of ways. One of it is when you made mistake. A good player avoid making the same mistake over and over that will eventually be punished in losing a match.
It is the same with life. You do not want to make the same mistake twice that will make you suffer the second time. That is the importance of knowing what was the mistakes that you made.
In tennis, mistakes happen a lot of the time. Learn from mistakes will make you progress better in tennis. But the skill to know mistakes to learn from is another level of coaching. Coaches should always ask question, especially open ended question to make the players think more.
It can happen during matches, during practices or while during free play. What mistakes the player made should be highlighted and asked to grasp their understanding of the things they did. Example of this maybe of the placement of balls. Why, how and when should be asked to the players in order to get them understand the decision that they made.
What happen if they manage to get a short ball? Should they approach? Should they hit a winner? If not, what kind of balls should they send to the opposite side of the net? While doing this, they will make mistakes by trying out, and learn from that.
It is about the tactical and also about the techniques they choose.
Ask correct questions, and players will learn more during their practice session. When players able to recognise their mistakes, avoiding the mistakes will eventually make them better players.