A common question I hear is whether EPUB3 is useless above and beyond HTML5. Why not just eliminate EPUB and use plain HTML5?
EPUB3 does bring new things to the table that HTML5 does not provide.
I see standards and technologies like ecosystems that respond to the challenges that are thrown at them. Before eBook systems, the web only had infrastructure for individual web pages that were either documents or application-like. HTML before HTML5 didn't even help much with applications, except for basic forms, so HTML5 provided features to help with this area (offline web apps, the canvas tag, etc.).
There has never been evolutionary pressure on HTML to have better long form reading for artifacts akin to eBooks, so it never really provided the facilities to help with this.