Ink dries quickly and it is waterproof. This is important if you are going to apply watercolours later. The image below is black ink. However, ink comes in different tones.
You can vary the pressure you apply to the Dip pen. As a result, traces will be thicker or thinner. This grants style and expressiveness. They are sometimes tedious since you have to recharge them quite often.
The trace rendered by stylographs (sometimes known as rapidographs) is uniform. Working with them is faster and easier. You can vary the way you trace by using stylographs with different widths.