How does that help?

It doesn't.

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.

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.

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.

... it was an ungrounded plug... Plus it was a chain, so it'd drape across all 3.

TBH, in the house I mount them ground down, but under cabinets or in the garage/shop or etc I mount it ground up.

I think ground up commonly indicates that an outlet is controlled by a switch on the wall. It's not code, but I think it's a convention

Agreed, I have seen that.

This is why you have modern circuit breakers.

Wow, I never knew they could be installed that way; the US standard doesn't say. Now every time I see a new outlet I'm going to check.

Or have sensible outlet design where prongs are always recessed.