Why do deaf people play badminton in the paralympics?

Actually any sport has hearing involved. You can hear the velocity of the ball going through the air, you can hear the sound as contact is made and estimate where it is going, you can hear the gasp of the audience as something happens unexpected. If you aren't facing in the right direction during a game you can't hear the sound of he game to know where things are happening. When you can hear, you have no idea how much and how often little sounds make a big impact on how you interact with the world.