Catwalks are usually found in theaters
and in industrial buildings.
In industrial buildings a catwalk allows you to observe or access a process. Occasionally, a catwalk is part of a tour allowing observation without encountering or interfering with the industrial process.
In theaters the purpose is usually to access stage equipment or a control booth. Whatever its purpose, a catwalk is primarily a structural element that also affects other structural components. A catwalk usually spans 20' - 30' and needs to be strong enough to carry whatever the equipment and occupant load that is expected. The supports for the catwalk are hangers attached to the structure above.
The examples below shows two types of catwalk used in a theater - the first detail is for accessing equipment, the second two are for loading counterweights onto scenery fly rigging. In each case the safety provided by the railings is tweaked to accommodate the job at hand.