Many reasons for me. One is that I have trouble with losing strength and having pain in my hands, so an ereader is a much lighter, trouble free way to read without straining my hands further, books can be heavy and awkward.
I love the convenience in many ways. No clutter/dust collecting in my house. As I'm a bit OCD it's thrilling to me that my books can be easily and quickly organized how I like and that I'm able to carry all of them with me. I also like to think of all the paper/energy it saves is good for the environment. I thoroughly enjoy the ability of shopping in the comfort of my home. I think it also must save time having whole libraries of books that you can sort quickly through search engines, preview, etc versus going to a book store and doing the same thing.
That's just off the top of my head, hope that helps!