This is one of those little things that makes perfect sense if you have ever had a common allen key brake nut strip out on you. There was a time when brake nuts all looked like this, then they were replaced with cheaper alloys, then the allen key hardware became ubiquitous. For a 1g weight gain, you can have your cake and eat it too.
What you get:
These are a direct replacement for the stock nuts that come with Kool Stop pads and many others. Deep enough to let you sale off the thin inner washer as well, and squeak another mm out of the tire clearance.