Where should I divide a mobile-friendly website from it's android counterpart?

I am creating both a mobile-friendly website and an android app for a client. Each will have similar functionality and objectives. As I began to create responsive pages for the website, the thought occurred of using the very pages that I was making as webview menus for my android app. Which lead to me thinking, is an android app needed at all?

My question is this What functionality can my android app provide that a responsive website does not?

The purpose of the app is primarily reading content(think ebook), sharing the content with others, and a link to buy the physical book. I believe that reading the content offline is justification enough for the existence of the app I just was wondering what else could I provide that only an app can offer?

My second question is From a design standpoint, should I use webviews for the main menus and content? Or should I just duplicate my responsive pages with android layouts/buttons.?

Webview based menus seems somewhat awkward, having to use javascript to run my android functions. However duplicating my web pages with android activities seems equally awkward.