iPhones are built with super tight tolerances. Using the same (or similar enough) materials on opposing sides means not having to deal with different thermal expansion properties. Glass is also radio transparent which makes the NFC radio and wireless charging much easier. It likely doesn't hurt with cellular and WiFi reception either. It's also not going to interfere with the MagSafe magnets.
Somehow Android phones manage to do NFC and wireless charging just fine without glass back panels. And tighter tolerances are exactly that - "premium look".
Tight tolerances are premium construction. Loose fitting parts are cheap and easy. They also lead to increased wear and decreased durability. Dismiss everything as "looks" if you want.
