Have you seen Andre Agassi play tennis? If you have then you know how he takes his shots on the rise. Meaning as soon as the ball hits, he returns it as its rising. This will rob your¬†opponent¬†from time. In otherwords; you surprise your opponent and you make him work a lot harder on the court to cover the return of all these balls.
Same thing in pingpong; if you get into the habit of hitting the balls as soon as they land on your side of the table; it helps you become a better player and be more aggressive player as well.