I think it's a bit of both. Glass is generally nice to look at and you can achieve certain colours or effects through reflection of light. An example of this would be the 'Honor 8'.
Another reason for glass is obviously for wireless charging (as other have mentioned), while there are ways to charge through metal, OEMs seems to stick with glass just to be safe.
Another is for better cellular connection. We see devices like the iPhone 7. They have to have plastic areas around the phone to let the cellular radio waves 'enter' the device
Glass is also not the greatest at holding heat, so you device will feel cool to the touch more often. Which some prefer. Maybe it's also cheaper to manufacture compared to aluminum or stainless steel?
I personally love the LG V10 back. We need more material devices.