Cookbook says to store dough in fridge overnight, why can't I just bake it already?
You can, in theory, leave it out on the counter for longer to develop the same amount of fermentation.  What you won't have is a very cold fermentation, which helps to develop the big holes in rustic breads that many people love.  Temperature and fermentation are a tricky balance, and you should listen to an expert like the author.  Also the overnight fermentation keeps you from needing to do something like a full 9 hours in one sitting.

Bikeboy389 made an excellent point in the comments.  BBA has absolutely everything you want to know to make its recipes successful in the first part of the book, which Reinhart strongly advises you not to skip.  If you want to make excellent bread, you will want to know all that information.  Plus, as a amateur or professional baker, you should find it fascinating.