Shock absorption is a good guess, however, this prototype has edge-to-edge glass, meaning that no matter where it lands, it will almost always fall on a vulnerable glass corner. And no matter how many shock absorbing seams they put into the side, glass will still be rigid, it's still made from one piece. I might be wrong, but I can't imagine such seams helping much when there is so much exposed glass on this device. Especially that the seams are on the sides (well, yes, they can't be in the glass, obviously), which only helps if the phone falls on the aluminium side, which is unlikely.