[fastball]

The screen on the (non-pro) iPhone 15 still has the 60Hz refresh rate, which is a limiting factor claimed by Apple, not just OLED.

Because in always-on mode, the refresh rate on the Pro drops down to a much lower refresh rate, as low as 1Hz.

Apple could've shipped an always on display mode with either OLED or a variable refresh-rate screen (or neither), but they only wanted to do it when they have both.

OLED so that black pixels are not illuminated and you save a good chunk of battery on that.

Variable RR so that you can drastically reduce that to save battery life as well.