Because for tennis, a sport that consumes a lot of energy, every time you bend down to pick up the ball will consume energy, amateur players or professional players in training, there is no special ball boy to pick up the ball for them, so when the re-serve directly out of a ball instead of bending down to pick up will save some energy. Another most important reason is that although professional players have a ball boy on the court to instantly run over and pick up the ball, if a shot is missed, the second shot must be completed within 20 seconds, otherwise it is considered a violation. Pulling the ball directly out of the skirt to serve not only saves time, but also maintains the inertia of movement and keeps a good feel for the ball. But if this ball falls out in the course of the game, the first occurrence requires a rematch of a ball, and the second occurrence will be ruled a direct score for the opponent.