Many people who do a lot of writing are dissatisfied with WYSIWYG word processors. What's the problem?
Working on-screen in something that purports to be what the final result always seems like such a nice idea, but in practice it just gets in the way. Fonts and layout that are good on paper rarely look good on screen, especially at tiny sizes (and even more importantly, why are you squinting your eyes to look at tiny text on the screen?). And even then you still have to print it out to see what it's really going to look like.
On the other hand, working in a plain-text editor doesn't give you enough. Sure you can edit raw HTML, for example, but who really wants to?
There is a valid reason to have formatting while editing, though: it gives the writer a subtle but clear indication about what semantic meaning has been assigned to different spans of text, and a plain text editor doesn't allow that.
But we haven't forgotten the real task most of us face: we have to write documents and whether they are white papers, technical reports, magazine articles, or book manuscripts, they are presented on paper and that's where they have to look good. That means careful attention to fonts, text layout, balance, and whitespace — all the things that don't work on screen.
Content, Presentation, Publishing
So what we are really want is separation between how our content is presented to us for editing, and what it looks like as published for others. We want a writing tool that is really nice to work in, that helps us easily navigate our documents and lets us get on with writing! And then we want a simple way to have that rendered into the output format.
And that's what Quill and Parchment are!