You are going somewhere and don’t want to carry the physical book with you, but would still want to continue reading it.
A function of the ebook software on your smartphone or tablet that allows you to point the camera at the page you are currently reading in your physical book. The app will recognize it and take you to the same spot in the ebook so you can continue reading.
You are reading a book and would like to discuss something in it with others.
An ebook reader feature that allows me to see if any others are also reading this book right now (and also filter those who have read the part I am on), and enables me to communicate with them regarding the particular part I’m reading.
Most books you read already are likely to sit on your shelf without and further use.
An app that lets you take a picture of a book (or the barcode) recognizes the book and shows you where you can buy it cheaply as an ebook. You can immediately purchase the ebook, and donate the physical copy on your shelf. You win by freeing space for new books, especially the kind that is much better in print (special editions, art albums, etc.), the people you gift the paper copy to win, and the authors and publishers win.