Well, just goes to show you how out of date I am! Though after reading up on it, I still don't like it much. It makes the web behave in ways I don't expect it to and I don't really see why anyone would really need this, unless they want to fundamentally change the way the web is expected to work.

It's main usecase is for making web apps work offline. This allows you to "install" a web app rather than a full-blown native app. If iOS supported it properly, then you could have truly cross platform, sandboxed mobile apps.

There used to be a declarative version AppCache, but in pracrice that made cache invalidation almost impossible to manage properly.

You can check "about:serviceworkers" on Firefox, and see how many sites install a worker when visiting their page. Spoilers: a whole lot of them

Mine shows none. Could be because of permanent private browsing. Do people still run browser on their personal computer without permanent private browsing enabled?

Do people still run browsers? I wouldn't know because I access the web exclusively using telnet.