When you have a 15x100mm thru axle fork mount, and you wish to transport 9x100mm bicycles, or you would like to give a better security to your 15x100mm front wheel to attach to an L1, L2 or L3 front wheel holder, the 9mm plugs are a great help to just that.
The set comes with a quick release, to secure any 9x100mm bicycle fork to the fork mount.