I have a lot of applications for entering text of various kinds into files but Scrivener is without a doubt the only one I would consider for any sort of long-form fiction. It's not so much that it's a distraction free environment, it's just a tool that's tailored to the job of writing fiction.

I wrote a novel in the app and the first thing I was able to do was capture series of notes on all the characters, locations, timelines, etc. This was invaluable for managing continuity and structure. Next I sketched out all the rough chapters. Between the notes and the chapter outline it was easy to write up quick notes in each placeholder chapter as to what major plot points, foreshadowing bits, etc needed to happen when.

After that was in place it was pretty easy to reevaluate and rework things until there was a clear arc, and then it was just a case of filling in the placeholders with actual text.

Could I do that in Word? I guess. I could probably have done it in a series of plain text files as well. You could make anything work if you put the effort into it. Scrivener already had the tools I need and arranged it in an interface that was easy to work with. It displayed the organization of the book in major sections, lists of chapters, and a separate series of related notes. If you can afford it I would highly recommend it.

The one thing that Scrivener did not do was provide an easy way to track and diff changes made by my editor. For that I had to export from Scrivener to Word, exchange the Word document, then manually apply changes back in Scrivener. That wasn't the best experience, but other than that it was perfect.