These products are useful. I like Scrivener because it can help me avoid the issues pointed out by
@David Y here.
I don't like having to open multiple documents to find what I need -- when in manic mode, the info must be close and clear and not distracting. That's the benefit of it all. Word is also prone to so many ailments (crashing if it's too long, corrupting for no known reason (and yes, the backups can also carry the bug and be unable to be resuscitated), and inserting hidden crap (like bookmarks that cause a doc upload to a site to fail)).
I also like to be able to separate out the scenes, or combine them, or view side-by-side with a single click.
I don't like to make silly mistakes, but it happens, so if there's anything I can use to avoid those issues, I'll be in it for a few tryouts. (The latest published story has a mistake in the TITLE! Gah! that's what comes of using/proofreading with Word.)
I've seen/read a book by a well-known, high-sales author who mentioned a character with a blue tongue and scaled lips in chapter 3, then in chapter 27 he has a pink tongue! Readers notice these things, so it's worth keeping each detail as close as possible when constructing the story. And editing.
Some people find Scrivener too steep on the learning curve, so other products may be worth looking into to see how they work for the way the writer works. I don't use all of Scrivener, but the bits I don't use I also don't miss. And it's not subscription -- pay once, never pay again.
Everyone will find what works for them if they want to go looking, otherwise, they'll just write (and some will keep it all in their head until the day comes when they have to remember everything about every character in a series like GoT!).