Everything I do is online, and as it happens that is the case with the majority of people today. Think about what you use your computer/laptop/tablet for, and I am willing to bet that most of it involves an online experience. More and more often people are using cloud services for everything. WiFi is everywhere and easy to access, even without a secure connection at home (which I have). The vast majority of browser based readers also include the option to read off-line as well, so if I happen to be somewhere without a connection then whatever book I am reading will still be available.
As I said originally, I want to free up space and be able to access my library from anywhere and on any device. I still plan on keeping my library stored on an external hard drive as a back up, but I don't want to have to carry around and connect to that hard drive every time I want to read something. Portability and universal access are key.