Using an application cache gives an application the following benefits:
Offline browsing: users can navigate a site even when they are offline.
Speed: cached resources are local, and therefore load faster.
Reduced server load: the browser only downloads resources that have changed from the server.
The FALLBACK specifies fallback pages the browser should use if a resource is inaccessible. Each entry in this section lists two URIs—the first is the resource, the second is the fallback. Both URIs must be relative and from the same origin as the manifest file. Wildcards may be used.
Fallback entries are used when an attempt to load a resource fails.