Graphviz was started about the same time that GUIs and bitmap displays began appearing. Over the years, we considered providing a GUI, but we never had the people resources to build one without heavily cutting into our research work on visualization algorithms. Check out the Resources page, which has some pointers to 3rd party GUIs for Graphviz.
In addition to the various layout programs, Graphviz provides a variety of other graph-based tools. One reason we are committed to a command-line interface is the ability to combine these various tools into pipelines or scripts to accomplish automatically much more complicated tasks on graphs.
That said, there is probably a need for a starter document that addresses these points, basically encapsulating what Steve wrote above, followed by a discussion of using Graphviz in pipelines and scripts, all with examples, followed by a pointer to the Resources page.