It doesn’t, from a technical standpoint. Existing phones won’t get it because that would take away a reason for millions of current OLED iPhone owners to upgrade. Can’t have that…Wait, why does AOD requires such advance hardware when pretty much any Android phone with an OLED screen has that feature for ages?
And don't tell me that the iPhone 12 and 13 won't get AOD because it requires the faster A16 neural engine...
Even in the Galaxy S7 days where there was no dynamic refresh rate and 14nm chips were the name of the game, the battery impact of AOD was <=1%/hr.