Tanner had a remarkable serve and was often noted for hitting it not only as it hit its peak, but while it was on the rise.
Hitting a ball it it's peak is recommended for maximum consistency and control of the serve because the ball isn't moving as fast (or at all) when it's at it's peak, making it easier to hit.
Hitting a ball on the rise is recommended for maximizing forward velocity because you gain the energy of the toss as well as the energy of the racket head on impact.
Hitting the ball as it falls is recommended for generating topspin because the ball moves down against the racket head which is moving up, so the energy of the toss is translated by gravity into rotation rather than forward velocity. The same principles apply in Table Tennis for high-toss serves.
In theory a player would select the type of toss and impact point which matched the type of serve he wanted to hit (high velocity, or high precision, or high spin), but in reality, each player is limited to the type of service stoke which is most natural to his/her body mechanics.
Very few coaches however, recommend tossing the ball much higher than the impact point because the unpredictability of wind conditions in tennis outweigh the value of generating spin due to a falling ball.