a brace definitely is not PURELY psychological, because there are some braces that distinctly do function as positive stops preventing hyperextension, as well as others that combine some anti-rotation features that are designed to protect the ACL.
a brace cannot be relied upon to protect all knee components 100% of the time, with that much I agree.
and well-trained supporting musculature is WAY better protection than any brace.
but for athletes who want extra caution and are willing to put up with the minor quarrels (muscles prematurely "pump," and quickness usually declines) the brace can provide some minor structural and MAJOR psychological support.
I've worn various iterations of anti-rotation ACL protection braces, ranging from a 1983-era Lenox-Hill custom to a 1999 BREG. I find that well-trained muscles are the best defense, but realistically speaking as one who sometimes has slacked on his PT/fitness, they are a decent safeguard if you are prone to joint injuries.
I am one of those loose-jointed people who is prone to sprains and tears. My last knee injury was a left ACL complete tear in December 1998, repaired in Feb 1999. I skied in the 1999-2000 season with braces on both knees, as it was my first season back on snow since about 1989 and that was only 3 years after a right ACL complete tear and reconstruction, in an era when skiing was at best done 1 year post-op. I skied in the 2000-2001 season for a few days with the left brace, but then quit using all braces completely. I haven't had any knee scares of any sort since returning to skiing in the 1999-2000 season.