It is related to BIOS machine level monitor and control of pc firmware including various sleep levels, power management during idle vs load, routine maintenance etc.  at user level it is not specific.  one cannot deduce much operational info from the brightness level or on/off state.  in old Dell AW A51 this power button color was programmed to flash slowly on and off to mimic winking of alien eye.  in that example the bios is written to make the power button led decorative.
 I keep my computer on 24/7 running folding[NAME]  when not in use.  Most of the time, the LED on the power button to my XPS 8920 desktop is off.  However, sometimes it lights up (bright white light), while occaisionally dimming or flashing intermittently before returning to full or medium brightness.  (this pattern reminds me of HDD access LED's on 90s computers).  My XPS has no HDD, only an SSD, nor do I see any unusual activity in task manager for any hardware resources.  This LED can remain on for hours at a time, and it's irregular- sometimes it goes a week without me seeing it powered on.  The manual only states codes for the colored error codes. Since this LED is off 90% of the time, what does it mean when it is on?  It can be annoying sometimes when it turns on randomly at maximum brightness in the middle of the night.
XPS 8920 power light- what does it mean? (not in manual)