I actually went backwards compared to most people:
From the OG iPhone to about the iPhone 6 Plus I went caseless. I loved it lol, some people thought I was nuts not having a cover on it. NEVER broke or cracked a screen. The phones did have wear and tear and I kind of liked it because it was somewhat of a 'patina' to the phone so it looks like it's been used.
The 6 Plus though... it looked a little beat up and I baby my iPhones. Tons of microscratches on the screen, and some medium scratches too. Aluminum was kind of scuffed and you can see where some of the impact/dents happened when I dropped it. I regretted not using a case and screen protector on that phone. It still worked, I still sold it to someone and got good money but yeesh I didn't like the scratches lol. I think it's because it has more surface area since it was so much larger than the iPhone 5's before it, that it being wieldy, theres higher chance of it rubbing, knocking, hitting stuff on accident.
So now I use glass screen protectors and cases. I used to think I was badass because I went raw without anything on iPhones but I realize nobody really gives a crap lol.
ALSO, this is a big one: case "technology" and design has also improved dramatically and it's almost fun to change up the look of your iPhone. No longer do cases have to be absolutely massive or made of only cheap plastic. A lot of improvements on ergonomics of these modern phone cases, some designs look sweet and it's reassuring that you can be a little rougher with it without freaking out too much or if you're caught in a situation where it could go wrong (ie: camping, at a party, at the park, etc)