It's easier than carrying books around, especially when you travel
It performs the other functions of a computer (e.g. web browsing, email)
They're a cheaper alternative to buying textbooks
It takes up less space than a bunch of books
The screen is hard on the eyes, unlike a book
I can read a book easier than I can read a computer screen
I like books and would rather have a book than a computer file
I'm a luddite and don't like new technology
I worry that I might lose such a small device or it will get stolen. With a book, who cares?
Something else (what)?