Since the days of Pirates! the thought of capturing the Spanish treasure fleet on its way back somewhere in the Spanish Main was a primary target because of all the gold and silver it transported to Spain.
Some of the colonies were rich in gold and silver mines. What seems strange to me is that Potosi was also rich from its mint and that the fleet not only transported raw metal but also coins made from that.
The Mercedes trove is on display at ARQUA. VALEROS
Spain’s hard-won shipwreck coins finally go on public display in Cartagena
The simplified usual pattern taught in school is that colonies mostly deliver raw material and the motherlands trade with manufactured goods. This seems the other way around.
That Spanish ships transport coins to Europe seems to be an unquestioned given.
So why didn't they ship exclusively this form:
Source: Atocha Treasure Coins
I didn't find any Spanish historical sources that weighed the arguments for either form and I did not find any concrete number that would make a comparison of the relation between pure metal shipped versus coins shipped feasible. That leaves me theorising and without hard number I am sure my imagined relation of both numbers is not even wrong by means of base-rate-error, as I just do not have any good rate or even representative sample over all. The anecdotal Nuestra Senora de Atocha seems to have carried 255000 coins but 'only' "over one thousand silver bars"
Why bother minting the metal in the colonies, increasing its bulk for transport, when the trade was tightly controlled and regulated? Would it not have saved shipping space to only load raw metal? Cargo space is a limiting factor and while it might not matter where a coin is minted, my current reasoning is that using the raw material for shipping it can be transported from every refinery directly to the port, whereas coins need an extra round to the less numerous mints and then to the ports. Refined metal in rectangular shapes should then also take less space on the ships then coins.
Is this more like "doesn't really matter with silver" or are there specific reasons?