My 6 year old was using my 6s plus and dropped it on the floor at a restaurant. The drop cracked the Zagg glass screen protector but no damage to the actual device screen (no case on phone at the time). Not sure it would have cracked without the protector but I don't want to find out. I purchased the same protector which was easy enough.
Screen protectors IMO also guards against those annoying surface scratches which generally can only be seen when the screen is off.