Before Scrivener, I used ywriter. The main thing is I need a program that allows me to move scenes around easily because I write very much out of order. I also hate the margins in Word, because I like the text to fill the box wall to wall; I like the split screen feature for continuity editing and shifting content around; I like the conversion to MS formatting (saves time) or to mobi files for beta readers; I like the notes and organisational structure built in; I like the Project targets feature and the wordcount breakdowns (scene, chapter, section, overall).
For my longer fantasy series I also need to write books 2 & 3 together, so the'yre in the same Scrivener project. Very long documents in Word get laggy as heck.
Basically, because I am extremely chaotic, I like my programs to do lots of heavy lifting in the organisation department. That said, my CP is very organised/more plotty than me, and she likes scrivener because it jives with her organised style. So who knows.
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