The switch from metal to glass is to allow Apple to bring wireless charging to the iPhone.
The front panel features a thin screen bezel with a space at the top of the display for a front-facing camera, microphone, and speaker.
No Touch ID sensor hole is present in the front glass, suggesting that Apple will be switching to a fingerprint sensor that's embedded into the display.
The rear panel of the iPhone 8, in addition to being made of glass, also features the protruding vertically-aligned dual-camera.
iPhone 8: What we think we know (and what we know for sure!)
Note: The reason why iPhone case leaks are common is because case manufacturers will pay handsomely to get their hands on a case early to get a leg-up over the competition. However, this demand for prototype cases also means that there are a lot of fakes flying around that have been mocked up by opportunists, so take every leak with a grain of salt.
Even if this leak is a fake, it is still highly likely that the next iPhone will switch from a metal back to glass, because current wireless charging technology doesn't work through metal.