It´s not that simple. Actually, the story goes about that – You are very talented (or at least You think, that You are) and You write a book. When You are done, You find out, that the book is too short or too long for it´s genre – cause (as obviously everybody knows) historical romance, for instance, should be between 100,000 to 125,000 words or otherwise no one will print them for very logical reason (yep, it´s money). So You have to edit Your book (what You´ve probably already done for ten times) and cut some of it or think of another chapter.
And why is it cheaper, when Your book is in that length? Well, Modern Matriarch says that “a number of factors determine the ideal book length and most of them are economic. From a manufacturing standpoint, books need to meet cost effective length requirements in order to be profitable. From a marketing standpoint, paperbacks need to meet certain length requirements in order to fit a predetermined number in one of those bookstore shelf pockets. Book size even determines how many books will fit in a shipping carton. And then there are consumer expectations. Readers are accustomed to certain types of books being of a certain length. Satisfying the reader’s expectations means your book has a better chance of making it off the shelf.”
That´s why it´s important, you twats. Gotta think financially.