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You don't, unless they pass it to you (as a return value or function argument).

Or if the borrow doesn't outlive the source e.g. if a local goes out of scope.

Return it from a method, or store it into a field of an object you already own.

I don't see that big a deal. In a language, say Ada, I would pass in an "IN" parameter, which cannot be altered in a procedure (and by default in a function). In other languages I would keep the scope/lifetime extremely small, and just not mutate or leak memory. This is just plain ol' good programming practice IMHO.

Is there another more compelling use case. In concurrent applications perhaps?

Yes, being able to pass ownership of a Vec to another thread is important, because otherwise the two threads have to coordinate over when to call its destructor.