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 ...

This time around it’s the glass on the back of an iPhone. It kept breaking and after a few replacements he wanted to try to replace the glass with a piece of etched brass. But part way through ...

Unlike the current models that use glass primarily for the front display and back panel ... similar to how it jumped from iPhone 8 to "X" (ten) for the 10th anniversary release.