The only purpose of that IMO is to pull the crank on the spindle far enough so that the stopper plate can be properly engaged. And perhaps a cover to keep your spindle from filling up with dirt and other crap.

You should use the one that came with your cranks.


P.S. It doesn't set the bearing preload. In fact it has nothing to do with the bearings and you can tighten it as tight as you can get it and you won't bother the bearings one bit. Unless you wrongly put a spacer on the spindle.