To me it's grip + not worrying about scratching the hell out of the back of the phone when I'm constantly moving it on my work desk etc.
My use of iPhones has always been low impact (i.e. mostly just office/home use etc) so I've never really struggled with damage, and have always been absolutely desperate to therefore use no case. I did actually do it last year for quite a while, but the downside was that a) it becomes super slippery to take out my pockets meaning I very nearly dropped it a couple of times, and b) when it was constantly up/down/up/down on my work desk I worried I'd scratch the back.
So when I saw these I immediately ordered one. It gives really solid grip (to me) taking it out my pockets, and I no longer worry about scratching the back. I had it on my iPhone 15 Pro for about 4 months and the sides didn't pick up any damage, but as above I know I'm a low impact user.
But... I 1000% appreciate that to probably 90% of iPhone users this is a 'pointless case' (not really actually a case I guess) and I respect that. Still thought I'd post in case it was of interest to some others.