Offline mode can be useful, mostly to who is online few times, but also if you just want to see a webpage again to read a problem solving guide, a new recipe, play a game or whatever it is. Google Chrome already has this feature, if you go offline it allow to open webpages that you already viewed loading them by its cache without need to enable offline mode, it's a kind of smart surfing configuration so the user hasn't to worry about enabling it each time it's needed.