This API is pretty useful for writing web apps that also work offline/with bad connectivity. It saves you from re-implementing the browser fetching/resource loading logic the browser already does for you.

It's very powerful, which also makes it a footgun: you can end up with fetch() requests going out over the network, with server responses saying one thing, but the frontend receiving something completely differently.

As for examples, I believe Home Assistant uses it to cache pretty much every resource in the frontend pre-emptively so you can use the web UI even if your internet connection is down (but your connection to your home server isn't).

We use service workers extensively in the kiosk app world.