I have used a case since the iPhone X. Why? Glass back.
Got a 8+ on launch day. At the time I never used a case. Somehow I developed a small hairline crack on the back glass. Phone was never dropped. Nothing ever bumped into it. Apple replaced it. Sold that when the X came out. Carried the X naked. Somehow developed a small hairline crack in it. Same story as before. nothing ever bumped into it. Never dropped it. Always set the phones down carefully. Probably too carefully. Apple replaced that as well. Ever since that started using cases and it never happened again. If it was something I did to cause the cracks, I will honestly say it’s ridiculous because I baby my phones. Especially when I don’t have cases on them. One of the reasons I like going naked is because I’m more careful with the phone.