Named for Sheldon Brown, who bugged QBP to have them made, and thank goodness for that. The deal is: Modern frames have recessed allen fittings for the brakes, which makes make you mount the front fender in front of the crown. For permanent use, that's fine, but if you remove fenders in fair weather, these Sheldon nuts let you mount in behind the crown, which makes on-and-off easier.
These nuts take the place of the stock allen nuts on most brakes. See the sizing: