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This is the first I've read of the glass back. Can anyone suggest a purpose for this other than making it easier to break?

Wireless charging.

Yep. It also looks and feels nice, IMO. The iPhone 4/4s had a glass back too, even without wireless charging.

Fwiw, you can do that with plastic too. It just needs to not be metal.

Then again, $1000 phone probably shouldn't be made of plastic.