If I remember correctly always-on displays were one of the features of OLED/AMOLED. OLED allowed software to only turn on the necessary pixels compares to the traditional panels that had to turn everything on at a time.
Efficiency+ flexibility
Discussion
OK, but what’s the usecase here?
on a smartphone? it's just a checkbox for the feature list. maybe some people like to keep the phone on a nightstand to double as a clock or have a widget they'd like to keep on the lockscreen with their phone on their desk🤷
I also think it is pointless 🤣 generally the phones that have that feature also look better under dark-mode if you're into that sort of thing; the power savings are negligible and its nice to not have all the pixels firing at you when using the screen in the dark