Like a portal, you mean? They can even be two physically distant domains with only a portal connecting them. But even then, why having the portal at all? I think it has a symbolic meaning, it is the representation of the choice to invite something...completely different, even if that means betraying your kind (sensu lato, anything from this side of reality). Or to leave it if you choose to cross to the other side. Just as Mab was not literally a mountain, a hurricane and I don't remember what other things Harry says, the Gates are not literally a pair of gates, the Pearly Gates are not literally Gates in Heaven, and the Apple of the Eden Tree is not an Apple.