The new generation of web apps are expected to continue to function when networking is not available for whatever reason. Obviously this is particularly important on mobile devices to close the gap between native apps and web apps.

At present it isn't possible to test the behaviour of an app when it is offline, or the transition between online and offline or vice-versa. This substantially decreases the utility of WebDriver for testing contemporary web applications. Neither is it possible to write testsuites for the features underlying offline support (AppCache, Service Worker), substantially increasing the chance of buggy or non-interoperable implementations. 

The most obvious way to provide this would be to expose an API to webdriver that would allow disabling "content" networking i.e. from the point of view of the webpage it would look like the browser was offline, but privileged code (in particular webdriver itself) would still be able to perform network operations.