It can work either way. The "traffic between industries" model is an older idea, while the idea of off-line staging is more in fashion lately. This is partially because contemporary model railroaders seem to be more interested in modeling a smaller geographic area, and most railroad traffic is over long distances.
This doesn't necessarily have to be the case. It is perfectly appropriate to model one industry that serves another, through the simple expedient of a view block. A classic example is a railroad split down the middle: an extraction-oriented industry is located on one side of the layout, and, totally hidden from view, an industry that processes the other industry's materials is located on the other side. Obvious combinations are logging site/lumber mill, coal mine/powerplant, rural grain silos/grain mill, etcetera. Even if you prefer a more mixed operating scheme, shipping between nearby locations is still perfectly fine, although in the modern age short trips are more likely to be made by truck.
There are advantages to using staging to 'offline' industries: for starters, you don't have to model them, or you don't have to model all the phases of a product. Also, some sorts of "offline modeling" is very appropriate to real railroading: interchange traffic, team tracks, LCL warehouses, or, in a more contemporary mode, intermodal traffic.