In the dawn of modern computing, there was only text. And the text was good. You can do a lot with text: write equations or sonnets, describe intricate computer subsystems or a fine spring day. But people are visual as well as verbal creatures, and there is simply no substitute for graphical communications.
Everybody needs diagrams. Most users need to create one more often than they think: that flowchart for a presentation, that sketch of the bird feeder to build this weekend, or a time line. Getting more technical, there are always circuits and blueprints and the like. Stop wasting time with an office app, the GIMP, or a paint program: use Dia, an easy yet powerful made-for-diagrams editor.
Don't let the simplicity of use fool you. Both Kivio and Dia, two free software diagramming tools, are very efficient at what they do. If you need to design a complex flow chart or create a no-fuss UML diagram then you could do a lot worse than to choose either of these packages. The tools have 90% of the expected functionality with only 10% of the hassle and fuss that more complex and unnecessarily feature rich proprietary diagramming tools deliver. The learning curve is small and the end result is potentially professional.