The endoplasmic reticulum (ER) is the site of synthesis of many proteins which will end up on the cell membrane, be exported from the cell or be sent to other specialised cell compartments.
All endoplasmic reticulum consists of a complex interconnected series of flattened sacs or membranes. Proteins are synthesized at the ribosomes. Many of these attach to the surface of the endoplasmic reticulum, making it look 'rough' under the microscope, hence the name.
As proteins are made on the ribosome, they are pushed into the sac. Here they are processed further, some are joined together with other different proteins whilst others are packaged for export or delivery within the cell.
Packages of proteins bud off from the ER and are transported around the cell.
In order to make complex products, such as a car, many smaller components are needed. In many cases, the individual component factories are sited together on industrial parks as it makes sense to make all the required pieces close together and then assemble them. Similarly, some components are made and processed in a factory next door.
The proximity of the factories means final products are made efficiently and can be packaged for delivery within the city or perhaps for export.