While Alexander took an expeditionary force off to conquer Persia, he left more than half of the Macedonian Army under Antipater to hold the kingdom. Antipater had many generals, and all of them wanted to win glory to match Alexander’s, fearing to be left behind or slighted when the king eventually returned.
As the Euxine cities were both fabulously rich and the source of grain for most of Greece, control of the Euxine would have given Antipater another lever to move Athens and Sparta. The timing of Zopryon’s raid seems to match Antipater’s war with Sparta, and it seems likely that Antipater’s first desire would have been to threaten Athens grain supply and keep Athens out of the war.
With 20/20 hindsight, it has become common for historians to mock the pretensions of “decadent” Athens and Sparta in the time of Alexander, but as an amateur historian, I’ve become convinced that Antipater’s wars in Greece were a “closer run thing” than Arrian would have us believe. Zopryon’s almost unheralded loss of an army on the Euxine and Leosthenes’ victories against Antipater show how little we know of the actual effectiveness of the Greek armies — and the capability of the Macedonian machine to fail.