One thing that surprises a lot of people when they learn about the Roman economy is that there wasn't really a unified coinage system until around 298 CE. That was partially due to practical "if it ain't broke, don't fix it" reasons. After all, as long as Rome itself received the correct amount silver in taxation, why would they care what Joeus Schmous exchanged to buy wine in Antioch after it was conquered? And besides, shipping small change across the empire is expensive, so why not just have the locals keep handling it and spare the expense?

But what might surprise you is not just that it took centuries for these local coinages to be phased out, but the number of such coinages increased for most of this period. Why? Partially it was a civic pride thing. We see inscriptions of cities bragging about having permission to mint their own coins. But it could also be profitable. We have inscriptions of laws specifically forbidding someone from using silver to pay for something priced in bronze and vice versa. You needed to visit a city run money changer, get your coins converted, and pay a percentage for the privilege. And you probably wouldn't have much of an option about that if you travelled to a new city. You had to physically carry pieces of metal with you if you wanted to move wealth around, and not only would doing so be a lot easier if you converted what you had to the much more value dense silver and gold coins, but if you were also from a locality that made its own local bronzes, there was no guarantee that anyone at your destination would accept them as payment.

Terminology is inconsistent for those coins. Older sources call them "Greek Imperial Coins," since the vast majority were made in Greek speaking areas and contained Greek inscriptions, collectors tend to use "Roman Provincial Coins," and modern academics tend to prefer "Roman Civic Coins." And what exactly counts as a proper official "Roman Imperial Coin" and something provincial/Greek/civic is also pretty fuzzy, as the central government clearly was involved in production of some of these coins sometimes. In the picture above, that small green coin is from Antioch on the Orontes,¹ and you'll notice it has the Latin letters SC on the reverse, something common for bronze coins from this city. This stands for "Senatus Consultum," which normally is used for something the senate of Rome ordered. Presumably this means there was some sort of formal decree allowing these coins or something, but we don't have any further record.

So what do I mean by "semi-compatible?" Rome still wanted specific amounts of silver, and they based this on their own coinage system. Those few cities allowed to strike silver would adjust their silver coins to convert easily to denarii (probably under Imperial direction). Like the one silver coin above is a four drachma coin from Alexandria in Egypt from the reign of Nero, and it had the silver content and value of one imperial denarius. However, one city's drachma wasn't necessarily worth another city's drachma, and if I recall correctly, Antioch's 4 drachma coins were trading as 3 denarii of value at this time. But in any case, this did mean that local bronze coinages did have specific exchange rates to the denarius. You might not be able to spend one city's assarion at another, but you might know that your local city could exchange 16 of them for a denarius, and you'd know that another city would take that denarius and exchange it for 24 bronze obols, both rates being pretty set in stone. And most silver could be spent over a fairly large area.

So why did this complicated system end? For the most part, you can see production of these coins drop off in the mid third century, almost all of them ceasing by the end of the reign of Gallienus (253-268). This was a period where the empire was producing a huge number of small, mostly bronze coins itself to its armies who were stationed all over the empire. A city couldn't just deny proper imperial coinage, and it'd be easy for it to overwhelm local production, particularly if inflation was high. General economic problems in the empire may also have made the acquisition of materials and the production of coins less affordable. The last hold out was Egypt, which had always had a semi-separate economy from the rest of the empire as part of Imperial policy since Augustus. But after a failed rebellion against Diocletian that ended in 298, they were forced to adopt the empire's standard coinage. Though strangely, when Anastasius reformed the coinage in 498, the mint in Alexandria put out different denominations than the other mints of the empire, so there still may have been some level of intentional separation that isn't obvious in the archaeological and literary record until then.

1. There were not only multiple Antiochs, but multiple that made their own local coinage in the Roman Empire. This particular example is from the most famous city with that name and probably the one you're thinking of, but the third coin on the top row is from Pisidian Antioch, a settlement in south western Turkey that was founded as a colony by Roman soldiers, which is reflected by this being the one coin here with a Latin inscription.