Lewis and Tolkien and Williams speak of the possibility of all myths being 'true' in some other existence than our own. Williams adapts the Arthurian Myth as a kind of objective correlative for his religious views.
They use myth in various ways and with varying degrees of effectiveness, but they have not really said why this is necessary. And to the extent that myth can be reduced to a set of rational propositions, this must strike the reader as making myth into something closer to allegory than to true myth.
But Barfield explains the origin and force of true myth in a way that the others do not. For Barfield, true myth is nearly impenetrable; because there are no 'ideas' in myth for the reader to penetrate to.
Myth is the closest thing in Man's mental life to pure pre-logical thought; meaning which the rational intellect has not yet ordered.
Myth is more of an experience than a 'thought': it is the form of unconscious meaning before the existence of any individual thinker.
Myth points back to pre-incarnate time when all that existed was un-individuated spirits...