It's been said before. Sony applies anti-shatter film on both front and back of Xperia Z. It helps the device survive fall damage ( look up youtube videos, it really does work ) but it's not scratch-proof, so Sony bundles another screen-protector which you can apply on top of anti-shatter film to prevent scratches to the film itself. You can apply it on top of anti-shatter film without worsening screen quality or responsiveness.
Now, you can remove the anti-shatter film pretty easily, and the glass beneath it is as scratch resistant as Gorilla Glass 2 etc. ( tested by Erica ) and it is still shatter resistant because the glass is tempered, but the film applied by Sony strenghtens the device so ideally it shouldn't be removed.
