An installation brief comes in many forms, from an interview to a written specification. It often does not come in a form that will enable the installer to directly proceed, without first translating or clarifying the job to be done. Even if the order comes with a written specification, then a set of drawings generally needs to be produced from it. This might seem a waste of time if drawings have not been asked for, but a lot of otherwise unavailable information about the hardware and the wiring can be derived from even a rough draft of the system diagram, although I concede that there are portable and straightforward installations that go together without a hitch or a single sheet of paper.
Block diagrams ...