How is a tennis ball useful in tennis?
The tennis ball is sized and constructed to work with the tennis rackets. It is pretty much essential because its handling determines the scoring.
Without a ball, you cannot have a legal serve. If you do serve an imaginary ball, the first volley will go on forever, with spectacular recovery shots made on both sides of the net.
Two opposing professional tennis players can go through a brand new tennis ball in under an hour. It really depends on the use of the tennis ball, though. If the tennis ball just sits around and is only played with a few hours a week at a non-competitive level, then the new tennis ball can last about three weeks until definite characteristics of the tennis ball have changed.