In fiction writing, writers create the plot, develop the characters, design the storyline, and construct each scene. Writers have stories they want to share, but at least in the first few drafts, the writers are writing for themselves.
As several of our experts have shared in their webinars, the transition comes when the draft is no longer written for the writer, but is being revised for the reader.
And this is where the editor comes in and begins working with the writer. Perhaps the editor comes on the scene before the writer has made the transition of writing for self to writing for the reader, but whether the editor comes on the scene before or after this transfer has taken place, this shift should occur for the manuscript to become a successful book.
In addition to the editor helping the book (and the author) in this transition, the editor corrects logic and grammar in the manuscript. And then finally, the editor is wrapping up the drafting process, finalizing the manuscript for its entrance into the world.