I just finished up a marathon (4-day) conference of medieval things. This was mainly a literary studies crowd, and I found it interesting to be one of the few scholars there not working primarily on literary texts. I talked about the way that medieval authors of historical tales used artificial marvels to comment on truth and authenticity in historiography and to stake their own claims to precedence and authority as historians.
One of the wilder aspects of medieval historiography is that there is a ton of freaky, bizarre, amazing stuff going on. Severed heads that tell the future, magical amulets, flying horses, subterranean cities, rains of blood, and sorts of other strange things show up. These things seem fantastical to us, but to medieval audiences, these kinds of phenomena differed only in their frequency, not in their inherent plausibility, from other, more mundane events. But marvels were important sites within a narrative where an author could explain or describe it in such a way to allow for a meditation on meta-narrative concerns.
But I haven't yet figured out *why* marvels, in particular, allow for this kind of intervention. Their inherent weirdness invites lingering over, and can also provide a moment for the author to reassure those who might find a particular marvel incredible.