My guess is that its strictly marketing. The reason I say that is there are many Android phones that have all these features and they don't use stainless steel.Yeah, I wonder about that, too. I think there must be a technical reason why. That the aluminum somehow interferes with the new camera system or with the ProMotion screen in some way I'm not qualified to understand? Or that the stainless steel provides some kind of technical benefit and not just a superficial one? I just don't think Apple (or any other manufacturer of pocket tech) would just use the heavier stuff just 'cuz, unless they had a too-light product that they felt SHOULD be heavier for user experience. They surely know how heavy the phones are, and how nice it would be for it to be a few grams lighter. So I'm pretty confident these decisions weren't made lightly or for just cosmetic reasons.
But what do I know? I'm a sheep. Baaaaaaa.
I 100 percent believe that if the rumors are true that next years Pro models are going to be made with Titanium that Apple is going to market the crap out of the point of their much lower weight.