You have to enter your email address to claim a book because a link will be sent straight to your email inbox. So, I need your email address in order to send your book.
The second reason is that if you should ever lose the book (PC, Phone, or Reader crashes/changed/stolen), you can log in on my site and download it again. You can also download the book on multiple devices.
Finally, there is this thing called Digital Rights Management (DRM) to protect against piracy. DRM works by creating a personalized digital watermark in the book, which states the name of the person to whom it was issued.