Originally Posted by
Kimmo
IMO it's more accurate to say it makes it easier to precisely adjust the headset by preventing the cup (or cone, depending on the design) from turning as the locknut is tightened, requiring a single spanner instead of two; if it's making the difference between the headset coming loose and not, the locknut is too loose.
No, that is incorrect. One should always use two wrenches. The purpose of the key on the washer is to prevent movement of one part inducing movement in the other, and to insure that proper preload is maintained when the locknut it tightened. The washer is not designed to take the stress of tightening.