I think I have finally found a use for Plato's damn Cave.
In brief, the Allegory of the Cave suggests that the reality we see around us is merely a shadow of some higher reality of Forms (platonic ideals), as people confined facing a cave wall with a fire behind them would only be able to see the shadows on the wall of actual objects in the cave with them.
Now, I think the Allegory of the Cave is a useless bit of self-congratulatory twaddle, when it comes to models of human perception in general, but it does seem to have some particular applicability to written fiction.
A good writer does a whole lot of world-building that never makes it directly onto the page. Just think about all the memes that go "name a character/fandom and I'll tell you X many things from my personal head-canon/mynon/etc about them". The words on the page are, if you will, the shadow of what the author built up in their mind, in their storytelling space. That building always has to be done; the author has to know all those little details. Without that, the shadow won't look convincing or have weight. But a shadow is still all that makes it onto the actual page. An author going to write fanfic has no direct access to the vast majority of the last round of world-building.
This means that, when writing fanfic, all that world-building must be done again from the ground up and it must be done in such a way that the text/shadow it makes on the page overlaps smoothly with the first text/shadow. This must be done without having ever seen the "object" that cast the first shadow, because that "object" only exists inside the first author's head. All the next author has to work with is the shadow.
When an author doesn't do the world-building, then they write a shoddy story. Fanfic or origific, lack of world-building detail results in an incoherent, inconsistent, or flat story. On top of this, when a fan author doesn't manage to make their shadow overlap sufficiently with the source shadow, then the story fails as fanfic. Fanfic does not have any of the work done already; what it has is an extra requirement in the building process. (This is, of course, modified by the fact that fan readers will forgive a whole lot of not-overlapping if the fan author still manages to give them sufficient id-candy tagged with appropriate names and outfits, just like origific readers will forgive a whole lot of shoddy world-building if the author gives them sufficient id-candy, period.)
So there we go: Plato's Cave of Condescension finally serves a useful purpose. Remix, reuse, recycle.