Substance graphs are the main type of graph created in Substance 3D Designer. Their purpose is to generate and process 2D image data that is not constrained to a set resolution, color or shape. They are meant as extremely versatile image-processing and generation tools, not just static, pre-set results.
The results can be in the form of a simple black-and-white pattern, a filter that only runs on other images and doesn't generate content by itself, or even a full-fledged procedural material with multiple channels.
Substance graphs are the most widely supported type of graph, and can be exported and used in a plethora of different workflows.