Dia can be used by many people in a variety of industries.
A systems engineer can create SADT diagrams for system analysis and design.
A requirements engineer can compose e.g. RE-KAOS, RE-Jackson or RE-I* diagrams.
A software developer can create UML diagrams of various types and extend them with the flexibility of a general purpose diagram tool.
A database designer can represent data relations with ER diagrams.
A control engineer may draw Cybernetics diagrams.
A computer programmer can create a flow chart to show the execution path of a program.
A network administrator can create a diagram to model the company's company network.
An electrical engineer may use Dia to create a diagram to show how a circuit works.
An mechanical engineer may design some Hydraulic or Pneumatic diagrams.