It fits into the notches on the nut on the negative side of the chainring and keeps it from spinning when you use a 5mm allen wrench to tighten the bolt on the other side. You can do without it if you grease the bolt threads and awkwardly use a tiny flat-bladed screwdriver to hold the nut in place, but this tool is made just for the task, and you’d be nuts not to get it.
