Could this be used to optionally reduce bandwidth use? For instance, could someone use this before going into Airplane Mode, even though they are in the middle of a city, but do not want to use wireless data?
Yep - that's one of the great things about service workers, IMO. The caching is pretty guaranteed, unlike the standard mobile caching which empties out far more often than you expect.
It is also useful for low-bandwidth or intermittent-bandwidth situations (i.e. poor network coverage).
yes, a site could allow the user to preload data into the cache and prefer that later.