Why should a tennis player "swing through" when hitting a tennis ball?

If a tennis player does not swing through, meaning they stop swinging the moment they make contact with the ball, they would lose the majority of their power. The momentum that they had built up during the swing is lost the moment they stop swinging, meaning that the ball is hit with a low amount of power.

If the tennis player swings through the whole time they hit the ball, then they keep their momentum as they hit the ball. There is a much higher power level when swinging through than if you were to stop your swing when you hit the ball.