I've been reading "The Bread Baker's Apprentice" and all the sourdough bread recipes call for overnight storage in the fridge. Is this really necessary? Can I just leave it out at room temperature and bake it that night? What is the purpose of storing it in the fridge for a night?
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.
A long, slow fermentation (known commonly as "retarding") of a sourdough bread is about flavor development. Yeast is most active at room temperature, so when you allow your sourdough loaf to rise overnight in the refrigerator, you're giving the bacteria that gives the sourdough its characteristic tang more of a chance to develop while slowing the yeast down. In my experience, the best temperature for flavor development is a slightly warm refrigerator, around 40° F / 4.5° C. (I have a separate refrigerator I use for beer and bread fermentation, so if you don't have a similar setup I don't necessarily recommend warming up the fridge where you keep Sunday night's chicken.)
As @justkt said, you can definitely let the bread rise/proof at room temperature for a shorter time. It just won't have quite as much sourdough flavor as it potentially could. To be completely fair, every area's sourdough tastes different due to the different local bacteria that take up residence in the culture. Unless you live in San Francisco, it's probably not going to get as sour as SF sourdough no matter how slow you ferment. Other techniques such as using a more firm vs. more liquid starter will make a difference too.
So, in short, feel free to rise the bread as fast as your little yeasties will work, but you definitely should give a slow ferm a shot at some point, and use the blue cheese and walnut variation of the sourdough recipe in that book. That purple bread will make your tastebuds sing!