There are those users who want to flaunt their device by keeping it open without any cases or shells, but what if they drop it....? The screen might be gone, so will the back glass part. Even tho it's gorrila glass. There are chances.
I have dropped my old Nexus 4 in hard concrete couple of times but the back glass(it was not gorilla glass either) never broke. Its all luck..
being said that gorilla glass has gotten better at this and is stronger these days and i dont think it will break that easily..