I have a "series 0" Apple Watch, ordered it the first day it was available, have worn it every day since, bump it into things occasionally. No case, no screen protector, and from a casual glance, you'd think it was brand new - no scratches on the screen (the lettering on the back is a little worn, but that's not the side I look at).
I've had iPhones since the 4, have always had them in an Apple silicone case (starting with the bumper on the 4 - I wish they still made those), have never used a screen protector on any of them. Dropped them a bunch, have never broken a screen, and they all look pristine after removing the case. On the phone, the case does two things: one, it means any scrapes from dropping end up on a replaceable case costing tens of dollars instead of on a phone costing many hundreds of dollars, and, two, the silicone cases make the phone phone more grippy, which means I'm much less likely to drop it in the first place. Of course, on the watch, the latter situation doesn't happen, since it's securely strapped to my wrist.
