Many athletes use elastic bandages and/or neoprene sleeves over and around the knee joint. There are different reasons for their use. Sometimes the athlete just wants to keep the joint warm and moving freely.
In other cases, there's been a minor injury and the sleeve offers support. Studies show these external sleeve supports can also improve proprioception. Proprioception is the joint's sense of its own position.
Proprioception is an important sensation to prevent injury. As the joint moves, it makes adjustments for any sudden changes or off-balance positions. Good proprioception helps prevent joint injuries. An injured joint often has altered or deficient proprioception. A sleeve can help increase proprioceptive input.
The sleeve does not change the joint alignment. That's where a brace fits into the picture. If your joint pain is from uneven alignment causing uneven joint wear and tear, then a brace to correct the alignment may be needed.
Some athletes wear both the sleeve and the brace. The combination helps improve proprioception while unloading the joint. The result can be decreased pain and improved function. For an athlete that can mean improved performance.