Transshipment is ordinary logistics. Most container traffic passes through a hub port that is neither origin nor destination, and the practice exists because it is efficient.
It becomes a control problem when the intermediate stop is used to break the documentary chain: goods arrive under one description and leave under another, or arrive destined for a party in the hub and leave destined for somewhere the original exporter would not have shipped. Free trade zones, where customs formalities are suspended, concentrate that risk.