There's a big overlap in all of them as they're not fully contained and separate from one another. I'll reference the 6 supported platforms by their numbers:
Service Workers (SWs) enhance performance for 3 & 4 and support a custom offline page for a better web experience. I think this is a good part of what it takes to support PWAs (platforms 5 & 6), so by supporting these features we're close to PWA level. The caveat of 5 is that Apple isn't welcoming to a good chunk of PWA tech (hasn't been and hard to believe it will change in the near future).
Note: This does not mean that PWA == SWs though, some good reads:
Because we've been pushing for a WebView approach on the native mobile apps (1 & 2) we also benefit from both the enhanced performance and the custom offline page from SWs. The most notable caveat is the reduced support in iOS, but some mobile native features are what gives both platform 1 & 2 their selling point IMO.
To answer the questions:
Without having context of the exact reasoning (at the time that it was implemented) I think it made sense. It gives an enhanced experience for people that don't want to install a native app. More favorable for Android compared to iOS though.
Support for users that don't want to install a native app at a low maintenance cost (most maintenance overlaps with what would take to support Mobile & Desktop Site)
Some edge cases come into play, the most notable I can remember is the Share feature that we want to enhance with native capabilities gets trickier to support for platforms 2/3/5/6. I don't think it's a big overall cost.
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Overview
There's a big overlap in all of them as they're not fully contained and separate from one another. I'll reference the 6 supported platforms by their numbers:
Service Workers (SWs) enhance performance for 3 & 4 and support a custom offline page for a better web experience. I think this is a good part of what it takes to support PWAs (platforms 5 & 6), so by supporting these features we're close to PWA level. The caveat of 5 is that Apple isn't welcoming to a good chunk of PWA tech (hasn't been and hard to believe it will change in the near future).
Note: This does not mean that PWA == SWs though, some good reads:
Because we've been pushing for a WebView approach on the native mobile apps (1 & 2) we also benefit from both the enhanced performance and the custom offline page from SWs. The most notable caveat is the reduced support in iOS, but some mobile native features are what gives both platform 1 & 2 their selling point IMO.
To answer the questions: