Because just putting your idea on paper isn’t enough. Rewriting is a necessity for any book.

http://madwomanintheforest.com/wfmad-day-18-revision-roadmap/

Some things I especially like in her post:

• Keeping track of times/dates. It’s easy to lose track of this when you’re writing, but makes an enormous difference to any reader.
• Mark locations. If half your book happens in a single room, then you might want to revise it.
• Control the emotional reigns of your story. If there is never a break for the reader, the book might be tiring. Jot down notes and plan your crescendos accordingly.