The purpose of supportive bracing after ACL reconstructive surgery is to protect the healing graft from excess strain. A functional knee brace or neoprene sleeve can also serve to keep the new tissue from stretching out too much, thus preventing joint laxity (looseness).
Many active athletes are under the mistaken belief that wearing the brace protects them no matter what they do. As you have found out, it's still possible to reinjure the ligament. Sometimes this is just a fluke. You move the knee in just the right direction with just the right amount of torque and the healing fibers give.
In other situations, the activity is too strenuous too soon. The healing fibers cannot withstand the force or load and retear. And early on in the rehabilitation process, you may not have regained enough neuromuscular control or proprioceptive input (sense of joint position) to protect that knee.
Wearing a brace or protective sleeve decreases (but doesn't eliminate) strain on the reconstructed ligament.