When a customer registers a Kindle to an Amazon account, Amazon links the device to all of the customer's purchases. Previously purchased e-books are available on the device, and Amazon links future purchases to the account in the cloud for later retrieval.
Amazon keeps records of Kindle book purchases in order to allow users to re-download lost titles or download books onto multiple devices connected to the account. Tracked purchases also allow Amazon to offer special discounts on multiple versions of the same book. For instance, Amazon may offer the Kindle version of a book at a discount to a customer who buys a physical copy of the book, Some Kindle e-books also entitle the purchaser to a discount on the audio version.