For sample-and-hold panel technologies like LCD and OLED, refresh is about updating the pixel state (color). There is a process that takes place for that even when the pixel data remains unchanged between frames. However, the pixels still need to emit light between refreshes, which for LCD is a backlight but for OLED are the pixel themselves. The light emission is often regulated using PWM at a higher frequency than the refresh rate. PWM frequency affects power consumption as well. Higher PWM frequency is better for the eyes, but also consumes more power.