Why ocean-going ships joined coastal convoys

Were ocean-going merchant ships diverted so coastal vessels could distribute their cargo?

Usually the ocean-going ship entered a major defended port and then joined coastal convoys to reach the port nearest its cargo's destination.

Major ports received the transatlantic convoys because they had the facilities and defenses for large arrivals. Individual ships could then be attached to successive coastal convoys and travel much of the way around Britain before unloading. This reduced repeated cargo handling and matched imports to regional port capacity.