Theory: They were masterpieces, in the old meaning
A masterpiece was an item you would craft or something you would do to earn you the rank of Master in your Guild.
This is easy to describe and judge, but hard to make if you aren't experienced.
They put them in their gold hoardes as they were basically their version of a degree
(If you're a metalworker and want to check if you would've earned the rank of Master in an ancient Guild, you can try to make one of these)