Looking up words on the fly by just holding your finger on them.

If the book is not written in your native language or you like to read books with unusual vocabulary (e.g. historical books), it's an absolute delight. So far, a concise dictionary like Oxford has worked the best for me, while Wiktionary or similar always came short.

The other is heft and handiness. If you read anything that is larger than a small notebook, an e-book is much more practical. You also don't have to hold it open all the time.