As C3 has the same game engine as Pharaoh, I guessed the same bug ocurs in C3, and indeed it does. When a warehouse containing goods is disconnected, the goods are no longer seen by the overseer. Also the traders don't see the goods. For now I can think of just two applications of the feature.
The first is importing more than the traders are willing to deliver. Apart from the maximum traders sell, imports are also limited by the size of your city and its industry. When you want to import more, put the goods in a disconnected warhouse, and the traders will continue delivering till they reach their maximum.
When exporting you can set the amount above which you export, but that is limited to 100. By having extra goods in a disconnected warehouse, that limit can be exceeded.
Disconnected warehouses don't need labor.
In Pharaoh the one tile distance is needed to provide fire and damage protection, in C3 that is not necessary. You may put the warehouse any place you like, it only needs access to the labour pool when it's active.