According to SetThreadExecutionState,
ES_DISPLAY_REQUIRED+ES_SYSTEM_REQUIRED Forces the display to be on by resetting the display idle timer once and Forces the system to be in the working state by resetting the system idle timer once.
ES_AWAYMODE_REQUIRED Away mode should be used only by media-recording and media-distribution applications that must perform critical background processing on desktop computers while the computer appears to be sleeping.
I suppose media-recording and media-distribution applications mentioned is just for explanation. The clue is critical background processing.