Unfortunately it is likely impossible to fully answer that question.
It is clear from the find evidence related to
cast coins, as well as many of the so-called
imitative struck coins, that they were not the product of foreign tribes or of simple criminals. The finds appear to indicate that they were tolerated in circulation. They are found in towns, on
military sites, in the civilian communities adjacent to
military sites, etc.
But beyond that, the rest is speculation.
We know those coins
met a need. But were they authorized by the central authorities, local authorities (remember that in many parts of the Empire local authorities coined local coinage until the mid-to-late 3rd century), local merchants, or simply by speculators? How much were they authorized versus tolerated? Did this change by region and time? (There were major waves of imitatives from the mid-1st to late 4th centuries and probably later.)
Evidence shows that one of the first waves of imitatives - Claudian
asses from
Britain, as well as
Spain and northern
France - likely originate in or next to
military camps. These coins were therefore almost certainly authorized by the
military to provide troops with small change. But it is unclear if this paradigm is paralleled elsewhere.
I recall reading somewhere, but forget where, that the late 3rd - early 4th century casts from
Egypt were likely made to make up shortages used when the centuries old tradition of striking local Alexandrian coinage ended in the 290s.
SC