The ground pin, when "up", is higher than the hot, so in certain situations it can prevent something from shorting the hot and neutral. Code (?) or convention requires it if you have a metal faceplate, and hospitals require it. People generally like them mounted ground down because then they look like little faces. :-)

edit: Not code, just convention.

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kstrauser an hour ago | parent [-]

Wouldn't it short hot and ground then, and still turn the necklace into a short-lived fuse?

The more practical reason to mount ground down is that wall warts with ground pins or polarized prongs nearly universally arrange them so that they're hanging down when inserted into a ground-down plug. If the plug's flipped, the wall wart's upside down and its weight is trying to lever it out of the wall.

	▲	linsomniac 10 minutes ago \| parent [-]
		Yes, in that case it would short hot and ground, which is effectively the same and hot and neutral, since at the main panel hot and ground are bounded together. But if it were, say, a metal credit card or something rigid, it might just fall on the ground, or could hit the ground and neutral.