Because nothing is stopping this app from updating its code after a few weeks in order to begin shipping data.
Of course, you can download the page and then use it off-line. Sure. But it'd be nice for a page author to communicate "I'm good, this page is self-contained and doesn't need any further access to the network, please let the user know."
Allowing a user to "pin" that status would similarly be great. Oh, and apply that or make it available to browser extensions.