The stroke lines can vary in many thicknesses and directions. The point of good line work is to focus on creating believable form.
You can use various strokes to create a multitude of textures.
The impact of different or heavier stroke lines can change the value appearance and textural appearance. The image of the lady on the left appears blonde and her hair looks like its light in weight. Whereas the lady on the right looks brunette and her hair looks slightly heavier in weight and texture.