Neither did I until I installed one. But it really works.
It happens because as you squeeze the brake, the braking forces bend the fork back and the brake bosses also down slightly, This lengthens the distance from hanger to brake. Longer distance means tighter cable and more braking. More fork bend. Eventually, fork springs back, shortening the distance and loosening the cable. One wobble cycle completed. Repeat until ... Mount the hanger on the fork and it moves down as the fork bends back (some of this bend is happening in the steerer above the headset). Little change in distance between hanger and brake.