Why preorder instead of just waiting to get it then? Well, firstly because you will get several free downloads of lovely pictures from the book for your phone screen or computer, and be entered to win a whole package of stuff including a vase and notecards and a Book Pin!
Secondly, if a bunch of people preorder it that makes everyone (bookstores, press, the publisher) even more excited about the book and they will do more to help promote it when it actually does come out. You know how it goes, if something is exciting to folks who are in-the-know then it becomes exciting to new people, too. And that would make me really happy, of course, so thank you! ;D