Why do you use a service worker?

[zwhitchcox]

I'm just curious why you would use a service worker as opposed to just regular javascript?

[TalAter]

Service Workers allow JavaScript code to run even when the user has no connection to the internet. This wouldn't be possible without a ServiceWorker.

You can read a bit more about how it works here: https://dev.opera.com/articles/offline-with-upup-service-workers/