It is a great example of knowledge work. Compare it with a manufacturing work on the photo below.
The knowledge work goes on inside a developer's head. This makes it difficult to intervene if a developer needs help on the work at hand. If a task is going off track, the developer can spend lots of time without anyone on the team even being aware.
At a system level, knowledge work is often interconnected: one knowledge worker's output is another knowledge worker's input. For example, a software tester will validate the overall software behavior after a programmer updates a software feature. Making the work visible helps the process of knowledge work as a whole.
The Card wall makes the knowledge work visible for Agile teams.
