Brush pens can be used to change the thickness of the stroke when writing by varying the pressure. This creates the characteristic brushlettering script.
You can also use a brush pen to color large areas, which can be colored much faster.
The special feature of brush pens is their flexible brush tip. By varying the pressure, they can be used to change the width of the stroke when writing, drawing or painting. A lot of pressure results in broad strokes and a lot of color. Little pressure, on the other hand, results in thin lines and little color on the paper.