People consume products in different formats.
Some people like reading, others listen to audio programs at the gym, others watch a lot of video, and others only learn when they are actively participating in the learning. By producing your product in different formats, you help your customer consume it in the way that most suits them, rather than whatever happened to be most convenient for you.
People consume products at different levels.
Some people don't have time to read an entire book, but would happily read a short e-book summary of the main material, and then dip into the book for the most relevant material. If you provide both versions, you help that customer get full value from the book. If you hadn't provided the summary version, some customers would have bought the book with the best of intentions, but might never have got around to reading it.
People consume some products with other people.
Some products are designed to be consumed in isolation, but others lend themselves to group learning. For example, if you write a book about sales techniques, you could include a collection of PowerPoint slide shows for sales managers to use in their weekly meetings. This not only greatly enhances the value of the book, it also means you can charge a much higher price because it could now be positioned as a twelve-month resource for sales managers.
Even a book, which is typically consumed by one person at a time, could be seen as a group learning resource. For example, if you added a postcard with key questions for discussion, you could make it easier for book clubs to choose your book for their meetings!
People consume products in different stages.
In this busy, time-poor world, it's becoming harder and harder for people to consume your product - in whatever format you produce it - at a single session. You can help them get more from it by also delivering it in stages. For example, you could invite readers of a book to sign up for an automatic e-mail course that sends them a weekly reminder about one concept in the book. Or you could conduct regular webinars about the book, connecting your readers with you and each other.
Don't ignore the benefits of product bundles!
A common objection to bundling products together in this way is the concern about duplicating your material. For instance, is it acceptable to provide an audio version of your book if it's the same content as the print version?
In general, the answer is Yes! Your customers will appreciate you taking the time to create the material in different ways, especially if you explain how they can use the different products to get full value from the package.