Have you ever tried to shut your computer down, only to have it hang on the shut down screen? Or maybe it’s locked up beyond any CTRL-ALT-DEL help. You may find that hitting the power button doesn’t seem to do anything.
If you really need to force a shutdown via the power switch, press it in and hold for 6 seconds. That should turn the computer off.
Only do this as a last resort—you should always try to shutdown your computer properly.