It's a matter of making the whole thing make sense in a tighter, more timely fashion. Whatever you do in one place affects the rest of the book. So if you change something you need to make sure that everything it touches gets the benefit of a re-look with that change in mind. When I wrote my first book I was primarily interested in seeing if I could get from the beginning to the end. Finishing seemed like the biggest challenge, and at the time, it really was.
All projects start off with enthusiasm.
Keeping moving when you have no idea when you're going to see land again, is a very real problem. At least it was for me that first time. Of course it had to make sense to sell, but first it had to be on paper, period. That was the first hurdle. The second hurdle is revisions.