A novel is not a very long letter.
Google Docs treats your novel as one continuous document, and for the first twenty thousand words that is fine. Somewhere past that, scrolling to find chapter nine, keeping track of who knows what, and reordering scenes becomes its own daily tax.
DraftProse treats the novel as what it is: a tree of scenes, chapters, characters, and notes you move around as the shape changes. The binder is the difference between managing a document and building a book.