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It's pretty much entirely a cool trick and a joke. It was framed as a social commentary on gluttony, but I doubt that was ever the real reason for it or taken seriously in any way. It's just a cool thingy.

I'm gonna say "for the fun of it" and "use for practical jokes" (choose your victim and context carefully!

Toilets work the same way! When water in the bowl rises to above the highest point in the drain, everything (hopefully) siphons out.

Science! pleasure can be found in something behaving in an unexpected manner.

for example, a cup that empties itself without overflowing the brim, but only if you fill it beyond a certain point.