What is monitor hardware calibration?
Hardware calibration allows you to adjust the monitor's internal image processing chip without changing the graphics card output data, keeping displayed images consistent with the original content without being affected by graphic card setting.
What does this mean in the real world? The calibration is recorded in the monitor itself (not your computer) so if you take your monitor and use it elsewhere the colours will still be accurate. It also means the calibration can be more sophisticated (and therefore more accurate) than when your computer is modifying the colours.