This extension detects if a website registers a Service Worker  by reading the navigator.serviceWorker.controller property . It also parses the Web App Manifest and displays the information in an accessible way. Find the source code of the extension on GitHub .
Why would you want this? If you aren’t into Web development, most probably you wouldn’t. However, if you are into Web development, the extension helps you identify (unexpected) Service Worker registrations in the wild and lets you analyze their code and learn from them.
Why would you use this extension? The answer is that the extension proactively detects Service Workers before you even have to open the Developer Tools (which you probably eventually will end up doing anyway).