paull:
It is the difference between the security folk who wave you through when your obvious metal belt set the alarm off and those you make you take the belt off and try again.
I set off the metal detector in a US federal building once, the guard told me to step back and try again, having described how to stop it tripping on my belt without having to remove said belt. I'm sure the technique would work for other small metallic items, but I bet he wouldn't give that advice now.