Provide hard line graphics in multiple colors. The CAD folks use them heavily still because thermal printers were generally too small, laser was too expensive and dot matrix was too low quality.
I got one of those setup directly above my desk. It's connected to an HP-85 with the plotter option ROM. It has a problem grabbing the pens so it will rather annoyingly error out from time to time. The pens are also drying out on mine. Wish I knew how to get a bit more use out of them.