This sounds silly but as an author myself it's extremely important that I know what the story looks like to a final reader while I write it. It's like dogfooding my own prose. Too many short paragraphs, or too many long ones, or strange gaps, etc. are much more evident to me in docx or scrivener or any other text processor.

I could never get over Markdown; Markdown doesn't show me the user experience of my own code(writing), while word and scrivener will show me instantly.