Because the goods being delivered needs to split into smaller, more specific routes as they get closer to the consumer.
Container ships are the cheapest way to get goods across vast distances, but once they reach a particular country, the goods will need to go in different directions to various different people and businesses that will buy them. Because, needless to say, container ships won’t deliver to your door.
So after ships, they go on to trains which take bulk loads of containers to other destinations where they will likely be split up more.
Also trucks, which can take up to a few containers at a time, take goods from ships and trains, and deliver them to more specific locations, such as a warehouse, where vans will then deliver the products to customers doors.
**Tl;dr: the process of delivering a container ship full of goods needs to gradually downsize until it reaches the consumer.**
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