Different brushes have different functions that make them suited to certain jobs. For example, a wash brush is good for covering large areas in colour and pastels or charcoal brushes are good for highlights.
For the portrait above I used several brushes. The rough sketch was done using the fine point pen and then I started to block in mid-tones using the wash brush. I added shading and highlights using the charcoal brushes. fine details like the hair were done with pen brush.
