1) You are not limited to specific eReader devices. Smashwords and a few others can download your selection and match any format for any device, including laptops and PCs.
2) Price. EBooks are cheaper than hard copies, even new releases.
3) EBook providers never run out of copies.
4) A virtual library at your fingertips. Download them wherever your computer can get online. I spent last winter in S.E. Asia and really had to search for book stores. The ones I found were full of the usual suspects: Stephen King, Grisham, etc. and local non-fiction on history and the wars that devastated the countries in the last 50 years. As long as you have a good signal from a Wi-Fi you’re set. Even some of the crappiest towns in Cambodia have Wi-Fi somewhere. I suspect a lot of these bookstores will die soon leaving the book dependent reader SOL.
5) Free books. Try walking into a bookstore and leaving with a free book; that is, one you haven’t stolen. There are thousands of free eBooks online.
6) Traveling. My Kindle and my laptop can hold a million books and take up little space in my carry-on.
7) If you have an eReader you can password protect rendering it useless to thieves. My Kindle looks like a diary or journal.
8) You can give reviews and express your opinion about the book, giving you more of an attachment to your favorite/hated authors.
9) Any book, any time.
10) Old and new books. Publishers are posting their entire libraries, some going wa-a-ay back. Indigo Chapters bookstores in Canada now returns books to publishers after 45 days. This requires readers to make frequent trips to the bookstore to find something interesting lest they miss it. As far as I know, my eBooks will stay online for a long, long time. The question begs, “For how long?”