Been using a case since my first iPhone.
3GS: the shiny plastic back is notorious to get micro scratches over time
4: I could probably survive without a case, but it became a habit in trying to keep the "shiny" last
5: case, since I notice the chamfered edges can be prone to scratches and dents
6s: case, since it is very slippery
Since most phones today use either metal or glass, which are prone to scratches/cracks, cases are pretty much mandatory, even on my Android phones. I miss the days of the original Moto G, which use a plastic back with rubber like texture. I never put a case on that phone. This is why a phone like the Poco F1 is quite refreshing for me.