This is either a precaution, or the knee doesn't give the stability they expect it too, when testing it. I know a lot of Olinemen wear braces, but that's usually out of being cautious, since they tend to have a lot of bodies get rolled up on their legs from the side.
http://www.wistv.com/story/29712236/clemson-qb-watson-will-wear-brace-on-repaired-knee
http://www.wistv.com/story/29712236/clemson-qb-watson-will-wear-brace-on-repaired-knee