I used tempered glass screen protectors for my iPhone 6 and X. I once dropped the 6 on an aggregate driveway and it shattered the screen protector. The actual display underneath was fine, but it’s entirely possible it would have been fine without the screen protector as well.
I just upgraded from the X to the 11 Pro and I’m thinking about skipping the screen protector. On the one hand I have worn Apple Watches for years without any protection and I’ve never cracked or scratched one (knock on wood). On the other hand the $10 Icarez glass protector on my iPhone X never bothered me.
EDIT: I remember another reason I started using screen protectors. I often carry my iPhone in pockets (pants, bag pockets, etc.) and don’t always remember when something might be in there that might be able to scratch the glass. I carry small pocketable tools around sometimes.