When an application is blocking power management, such as a video player, the screen will stay on and will not automatically lock, which is good.

However, if the screen is manually locked it will stay on indefinitely (until the inhibition is released) which is pointless as you cannot see the video anymore either and you can't do anything with the computer anmyore. When the screen is unlocked, the original policy comes into effect again.

I'm listening to music all day here and when I leave my laptop I lock it manually but the screen stays on forever because of the music playback.