Typically most web-based applications will work only if you're online. But HTML5 introduces an application cache mechanism that allows the browser to automatically save the HTML file and all the other resources that needs to display it properly on the local machine, so that the browser can still access the web page and its resources without an internet connection.
Here are some advantages of using HTML5 application cache feature:
Offline browsing — Users can use the application even when they're offline or there are unexpected disruptions in network connection.
Improve performance — Cached resources load directly from the user's machine rather than the remote server hence web pages load faster and performing better.
Reduce HTTP request and server load — The browser will only have to download the updated/changed resources from the remote server that minimize the HTTP request and reduce the server load.