It is basically reinventing caching but correctly, caching at the http layer does not handle updates properly, preloads very well at all and is entirely unreliable.

For example load http://pouchdb.com/, first page load may be a little slow but the rest will be very fast aside from the api page which has some slow JS (if the rest of the pages didnt load instantaneously we would probably never notice how JS slows that page). Turn your internet connection off while you browse, its all fine.

The above is using AppCache, but a big part of ServiceWorkers is learning from the AppCache mistakes and having a more flexible / powerful framework for them.