I haven't done CBL but did do a CKG (basically similar to carb nite).
Kiefer says the reason for the prep phase is to prime the body for using fat as a fuel.
With my experience on a CKG, my energy levels were completely different from day 1-2-3 cold turkey no carbs and after staying the course for one or two weeks. There was an clear and obvious change in how many body was managing its systems.
I would follow the book and his reasoning.