At one point in the film, Annie asks: "Why does it always have to be 'once upon a time?'" and she's right, and this is a film about how she is right.
Annie defines the phrase "quotidian world" -- as opposed to the world of the imagination -- as "the one we have to live in." But that's just it: We don't have to live in it, at least not all the time. Selkies aren't the only creatures who can inhabit two worlds. As Annie knows, and as Jordan's film makes clear, stories enable us to step outside the quotidian world and dream, if only for an hour or two.