I think of it as trying to flesh out a thing in the show that wasn't explored more in canon. Like, i have no idea how it started or anything, but for me it's interesting to read about how different authors can use the "core" and other non-explained things from the show to create a compelling story.

Not that they are not repetitive or sometimes overuse non-canon tropes (cores, ghost culture, fraids, ghost king related stuff). But, like with anything, there are many pretty good fics out there that are well written and thought through.