The last page in your book must be blank. If it isn't, Ingram will add a blank page. Ingram need a place to print some manufacturing information — including a small book barcode, and a production tracking number.
The book barcode helps marry your book with its cover. These parts of your book — the book block containing all your pages, and your book's cover — are printed separately and glued or bound together later.
The production tracking number allows any book to be traced back to the printer it was printed from, anywhere in the world. Misprinted books are rare. But if it does happen, a tracking number lets Ingram quickly find and fix the fault.