A system diagram is a visual model of a system, its components, and their interactions. With supporting documentation, it can capture all the essential information of a system's design. There are many variations of diagramming style.
Diagrams, if properly constructed, are excellent mechanisms to communicate complex information to others, much like a well-made technical drawing. Designing is, among other things, an act of communication, so making sure one has an accurate and relatively easy to read account of design decisions is important for communicating that information to others.
Produce a plan, simulation or model.