[jonbronson]

Sure, ultimately most rendering is about producing 2D images as output. But the whole OpenGL pipeline is designed around taking 3D data, and providing abstractions to enable that rendering. Rasterization and depth sorting are only one piece of the rendering pipeline. Of course it's also possible to drop the z coordinates, or use orthogonal projections, to use OpenGL in a purely 2D way. Plenty of applications do that, but that doesn't make the API 2D. It just makes the use case 2D.