Personally, over the years, I collected large amount of e-books. And why I'm keeping them all? Because I want to return to some of them from time to time, it's good source of knowledge (even though only a small part of them are educational books) and why not keep them all if it cost only few hundreds of megabytes?
I can always carry my entire e-library with me on a single memory card! I just moved to another house and the weight of crates with all my printed books almost killed me! I still feel them in my back!
And although I still purchase printed books, I preserve only the best of them and my main library is now e-library