Knee braces designed to support and restrict the knee joint are often used after ACL reconstructive surgery. The main goal is to decrease the strain on the healing and recovering ligament. A supportive device may also improve joint proprioception (sense of joint position).
Some surgeons use these routinely with any athletes who are eager to get back into action. Others do not see the need for a rigid knee brace but suggest using a neoprene sleeve instead during strenuous activities. The knee sleeve provides compressive support but has an opening for the patella (kneecap). It allows unrestricted movement.
And there are those experts who believe no support is even needed. There are some studies that show no difference in outcomes for patients with or without support. It's possible that there are subgroups of patients who need a brace and others who do not. And it's even possible that certain groups would do better if they wore one supportive device over another.
More studies are needed to find out who can benefit from knee support and specifically what kind is best. This may depend on the age of the patient, the length of time from the injury, sex (male versus female), and type of reconstructive surgery that was done.