Outlines and worldbuilding are very important to the process of writing a book, and most especially when creating a series. Though most of the material is never used, the information garnered is essential. These tools are what keep the story on tract, the underlying structures. Thus, influencing how the story unfolds, how the characters interact with one another, and keep the scenes and dialogue true to the storyline, so there's no meandering, no situations that crop up that are not pertinent that could only serve to confuse, frustrate, and bore the reader. Death to any book!