The epilogue is because I want to tie up a few loose ends, but they fall after the main plot and character arcs of the story are finished. That's why it won't be part of chapter 17 -- because it would dilute the focus of the story. This way, I really could just leave off after chapter 17, but the epilogue is... hmmm, how to put this? It's like an extended version of the traditional "And they lived happily ever after" sort of ending.

I generally have a rough word count estimate, but I am flexible about it if the chapter has a particularly good end point that makes it shorter or longer than my goal.

I divide into chapters and scenes for the same reason I divide into paragraphs: readability. It's visually hard to decipher an unbroken block of text, and it's also mentally hard to arrange huge chunks of information. So we break it down, leaving white space between paragraphs (or indentations, depending), and splitting the action at various points. It makes the story flow better, makes it easier for readers to track what's going on.

(Also, finishing a scene gives me a little jolt of satisfaction. Finishing a story is more satisfying, of course, but I like to have little boosts along the way, because writing a novel takes a long time, and without any rewards on the way, I'd get sick of it quite soon.)