Wiring ducts are long hollow structures meant to carry one or more cables inside.  They are usually made of metal or plastic and can vary widely in a cross-sectional area depending on the volume requirement of the application.  The ducts serve as both an organizational tool for multiple wires traveling in the same direction and as a protective shield around them.

From a protective standpoint, ducts can be useful in many aspects.  They prevent the wires from incurring any physical damage from mechanical trauma or wear.  This can include foot traffic, tool damage or any other situation where physical damage can take place.  Some ducts are completely sealed off from the environment, so the internal core is protected from any negative environmental factors such as heat, moisture, solar radiation and dust.

Organizationally, ducts can provide a common path for a grouping of wires, much like the task a cable assembly performs.  Wires can be grouped by purpose, need, destination or any other effective way.  They also allow wires to be routed easily in any location, be it the ground, wall or ceiling without the need for additional support or cable clamps.