I don't see what any of these various softwares do that MS Word (or a mere pen and paper) couldn't.
It depends mostly on how you like to work. As an ex-programmer, I need some structure, and the easier, the better.
yWriter4 gives me that by making it easier to organize by chapter and scene than Word. My chapter/scene outline is always right in my face, so moving from one scene to another to check continuity is fast. I can keep notes about characters, locations, or items and access them with a click or two. It's easy to drag a scene from one chapter to another.
It's easy to store a short description of each chapter and scene along with the main text, and I can print either of those sets for a synopsis or overview. I can set a scene as unused and it won't clutter up my printouts. I can still get a printout of a scene, a chapter, or the whole thing, with or without all the notes. I can export the whole mess to .DOC, .RTF or HTML if I need to, so there's no problem with portability.
When I used Word, I could still do all those things, it just wasn't as easy.
I got used to those organizational tools when I wrote software, and I really missed them when I went from writing code to writing novels.
Actually, I didn't realize until writing this post that without my programming background, I probably wouldn't have known how important those tools are for maintaining a big project. There are a lot of parallels between writing software and writing a novel. The programming environment I used and yWriter4 have more in common than I had realized until now.
YMMV.