Having just read about this a bit more, it looks like it is also to prevent something falling onto a half-inserted plug and causing *sparks*. China's "upside-do

Having just read about this a bit more, it looks like it is also to prevent something falling onto a half-inserted plug and causing *sparks*. China's "upside-down" version is also a solution to this, in that something falling would fall onto the earth pin first. Not sure how that works for two-pin plugs.