Other times, a different yet equivalent part is used. We’ve all replaced drain plugs or lug nuts that take a different tool than the factory required, due to the relative interchangeability of those parts. In other cases, I’ve gotten hardware that’s not held to the correct manufacturing tolerances, so an in-between size from the “other” drawer (metric on a standard fastener or vice-versa) is actually the correct tool to use to protect the fastener’s head.
After talking to the fellas about this and reconsidering my own tools, I had three major takeaways. The first is that you really don’t know when you might need a strange size, so having it usually beats not having it. The second is a realization: the tool companies aren’t dumb. If they make a tool for it, it’s because a fastener exists that requires it. Keep fixin’ stuff, and you will run into it eventually.
My final takeaway should come as no surprise: the 10mm is, for a lifetime mechanic, quite literally worth its weight in gold!