I'd say part of the reason is to speed temp recovery because you are opening the lid to check doneness every 1/2 after you up the temp. Part is probably to get a 'crust'.
Never worried about increasing the temp at the end. Depending on the cook, sometimes they are close to done. Mkae the call at the turn whether you think the ribs need more heat. Sometimes I've just needed to get open the vents because my fuel is running low.
Its a great recipe, but better yet, great guideline at how to turn out very good baby backs consistently.