Hold on, folks. The fault may not be at an endpoint. Consider a fault inside the sub-panel, i.e. your L1 feeder wire frays against the sub-panel case. It's before the sub-panel main breaker, so no help there. Without a wired ground path back to the main panel, the main panel breaker would not trip, as not enough current would flow. (earth is not a great conductor, so grounding electrodes hammered into the earth won't serve as a high-current ground.)
So without a wired ground, the 'ground' in the outbuilding would simply float up to 120v. You'd have 120v on every ground pin and metal part of the electrical wiring, and any equipment which grounds chassis. (neutral would still be neutral - remember they are isolated from each other in a sub-panel - so equipment would still work normally.)
Of course your outbuilding 'ground' (now hot) would hunger for a path to main panel ground, and it may find one. As long as flow is less than 100A (i.e. resistance is >1.2 ohms) it won't trip the main panel but will make heat - up to 12,000 watts of it - in places you probably don't want heat!
Or it could be seeking out parallel metal pipes and causing galvanic corrosion in them, and it could potentially do this for a long time. Which can put gunk in your drinking water, as Flint learned. Streetcar systems, hastily built, used to have a big problem with corroding parallel gas and water mains until they buried their own "ground feeder"... in other words, exactly what we are talking about here.
Anyway, to answer your question, current goes through all the breakers that it goes through. A fault is simply more current flowing through the "hot". Ideally enough to trip a breaker, but not necessarily. We try to help that along by providing solid current paths for faults to take.