In an enterprise environment there should be thousands if not millions of cables running around for data connectivity. Managing all those cables can be very frustrating if you don't carefully design the cable placement and everything else.
That's why most modern buildings nowadays have cable trays in place just on top of the ceilings. The reasons to use cable trays are obvious, to keep the cables from tangling and to keep cables used for different purposes away from other cables, e.g. networking cables away from electrical cables to prevent interference.
The pictures above show how the cables are arranged at cable trays, you can see that the cables - gray cables for UTP cables, blue cables for fiber optic cables, and black cables for electric cables - are separated and arranged.
In reality, sometimes you can't easily arrange them like that, because the cable installers/contractors are commonly different for networking cables, electrical cables, and even for the cable trays installation. As network installers, we surely want to keep our cables away from the electric cables, while other not-so-professional installers or contractors won't even care of it. The point is to always keep smooth communication with other contractors within a project.
The cable trays carry the cables from the telecommunication rooms to the end users, you can see the exposed trays like at the pictures above typically located at the telecommunication rooms. While the trays to the end users are covered on top of the ceiling.
Below you can see the trays heading to the racks, just imagine to arrange all those cables.