Knee Support got two most common type of sleeves. One is without hole and another sleeve with a hole in it. The knee sleeve serves two major purposes. The without hole one that it provides compression (if it's made of compressive material). This is helpful to manage mild swelling around the knee and keep blood flowing to the muscles that support the knee (quadriceps). Its also provides a significant amount of proprioceptive feedback to the brain, helping to keep the body at a heightened level of awareness that the knee might need a little extra help.
The knee sleeve with a hole in it, is designed to support the kneecap. Certain conditions will cause the knee cap to track improperly, causing irritation around the knee joint. The hole in the brace will encompass the knee cap and help to keep it on track.