Those other sites do it so that they're available offline. Service Workers give you control over network caching so you can return an cached/offline copy of the content.
I remember when this was actually a feature of web browsers. In the File menu you could just click "Offline mode" when you were offline, and it would return everything from cache. And then one day that menu option was just gone. Now the web has an opt-in way for sites to do this themselves. It feels pretty convoluted. Then again Web 3.0 feels pretty convoluted in general. It's as if the iPhone SDK came out and suddenly set the standard on how convoluted development ought to be, and every other platform followed suit.
Firefox has it. Menu > More > Work Offline.
Safari has a similar ability called "Reading List". And even saving pages as a webarchive. The problem with both are that modern websites depend so heavily on XHR that caching the initial page assets is essentially absent of the content desired, and thats what service workers can help fix.
It's like that functionality has just been forgotten and then this generation's hip coders thought it up again in their convoluted "everything is dynamic and JavaScript" mindset. Like with 200 other things regarding web browsers.