Why do tennis players wear tape below the knee?

Answer 1

Tape is used on vital joints whilst performing strenuous exercises to prevent sprains.

Another view:

Taping is often used to prevent injury or it can be used to stabilize a current or recovering injury. Most often you will hear of ankle taping, but there are taping methods that can be used on almost very joint. There are also methods of therapeutic taping that help to relieve pain and/or assist the body in movements.

Answer 2

http://www.physioroom.com/how_to_guides/patella_tendon_taping.php