Creators don’t like people saying that only 1 episode matters. The creator says, it all matters (They also tell the picky fans to get a life). Scope. Some like their series to never end.
One part after the other that continues the story on and on.
Obviously, creators enjoy setting up a narrative, a propelling, never ending scope for their story. It’s the major arc. Creators love the thrill of filling in the detail and working out the finer things that link to the next episode and the next one and so on. Hard work, but thrilling. Creators love series and so do audiences, but a few like the story to finish in one film, book, or whatever medium it’s in.
We don’t live in a publishing world where one book/movie about one thing is all there is. There can be several stories about one thing. For example, stories about adventure, with an angle. Or what happens to the boy who gets lost down a chute? Does it lead him on adventures in another world?
I reckon the nature of publishing is the scope of the key character, the wider story, a general or specific theme. Where does it lead? What’s going to happen? It won’t necessarily end.
I think quite a few commercial publishers want to know that prospective writers/authors have something more in them than just one book about their central character, theme, story, etc. They want to know if your book can be turned into a series or there is potential to do this. But first show them you can write the story brilliantly.