Maybe someone who knows more about AMOLED panels can comment about the role of PWM? It seems to be something other than brightness control, the article at [1] says:

At any level of brightness, modulation with a frequency of approximately 60 or 240 Hz is present ... It can be seen that the amplitude of the modulation is not very large at the maximum and near to it brightness, as a result there is no visible flicker. However, with a strong decrease in brightness, modulation appears with a large relative amplitude, its presence can already be seen in the test for the presence of a stroboscopic effect or simply with rapid eye movement. Depending on the individual sensitivity, such flicker can cause increased fatigue."

[1] https://translate.google.com/translate?act=url&depth=2&hl=en...

PWM is used to control the brightness level of each pixel.