Diagram use :
Data visualisation: as most diagrams are created by a person with a com- municative intent and an understanding of the expected reader, data visualisation diagrams help with the characterisation of data and formalisation of design rules in order to facilitate the user (e.g. in helping a programmer to write code).
Graphic design uses diagrams in order to facilitate the task of a graphic designer to accept a given set of information, who can then prepare a way of communicating that information effectively.