1. Space -- I have an entire closet full of books. Every time I buy another physical book, I am slowly crowding myself out of a little more space in my apartment. Electronic copies take up no physical space.

2. Can read anywhere (using my phone). I don't like this for every book, but some books are good for reading in little bite-sized chunks. I read a page here and a page there while I'm taking a break at work, waiting at the post office, whatever. The constant availability makes it a lot easier to increase my reading bandwidth vs. trying to always have a physical book on me. Plus variety--I have hundreds of books in my pocket at any given moment, and can choose whatever I'm in the mood for.