The biggest advantage is immediate fulfilment, coupled with some simple copyright controls. Once purchased, the books are “marked” to identify the purchaser. This of course clearly assigns the book as the purchaser’s property — and obviously discourages the purchaser from redistributing the book without authorization (such as uploading onto a free upload or torrent site.)
You can continue to order the book in hardcover from Amazon.com and other book retailers, as well as directly, and electronically through Amazon, the Apple IStore, Google Books (Play) and other e-retailers. Retailers can set their own prices, so you’ll find different offers.