As soon as the enemy loses a zone base, all of his bases that are tied to this zone base have considerable longer downtimes for everything that's usually been supplied by the convoy system (Ords, Dar, etc). That's because the according strat factories have changed their ownership.
Of course that works both ways. The coutnry that just captured the zone base has now a source of supplies automatically distributed by the convoys - reduced downtime at every field belonging to that zone base.
EDIT. Slapshot beat the Snail to it