.....a reason for it to exist.....
Keeping this in mind, you have to understand which commodities are carried. Steel plates and sheets, for example can be used for automobiles, railcars, and even regular construction. A concrete plant, for example, can be used for making pipes and cinder blocks. These industries also need incoming materials which would be iron ore and coke for the steel mill for example. A lumber mill would take full trees and produce board lumber which can be brought to a factory to make furniture and other goods.