Apple started using glass backs starting with the iPhone 8 because of the wireless charging coil. That's the first model to have the wireless charging coil. The reason why it needs a back glass is ...

Apple introduced a glass back to the ‌iPhone‌ with the ‌iPhone‌ 8 and ‌iPhone‌ X in 2017, but prior to that, every ‌iPhone‌ except the ‌iPhone‌ 3G, ‌iPhone‌ 3GS ...