For example, you've hit another car (possibly an expensive one) and caused tens of thousands of dollars in damage. Or you've inured a passenger and caused hundreds of thousands of dollars of hospital bills.
The kinds of events that would financially ruin you, or at least seriously set you back on your life goals. Or, in the case of state-mandated insurance, where there's a high likelihood you wouldn't be able to make the other party whole.
You can use it on smaller things if you insist (and your rates will go up), but that's not really what it's for.
Personally, both the events you described fall under my definition of one-off maintenance ("things that are an unpredictable yet unavoidable cost of owning a vehicle") and that I'd expect to pay for out of pocket.