Tennis players are often seen tapping the ball a few number of times before serving. It There are many reasons why players tend do so.
Firstly, some players check the bounce of the ball before serving. This gives them an idea as to how much power should they strike the ball with. Secondly, for some players it is simply a pre-serve routine. Tapping the ball before serving becomes a habit over the years and players thus repeat the action each time. Thirdly, some players believe that the tapping of the ball helps them get into the rhythm before the point begins. Finally, there are a few extremely rigorous players who believe that they bounce their ball in the area where they expect their ball toss to be.
There are also a few tennis players who do not have the habit of tapping the ball before serving. They simply hold the ball in the triangular socket between the head and handle of the racquet before serving. In complete contrast, there are a few players who tap the ball incessantly much to the frustration of spectators and the opponent. This has been now restricted as the shot clock has been introduced. If players exceed the time limit between two points, then they are given a time violation.
Tapping the ball before serving can come in handy if players are about to surprise their opponent with an underarm serve. During the act of tapping the ball, players find it easier to swing the racquet from below (underarm) which often stuns the returner.