Adobe Illustrator brush tool is great in that when you draw something, it smoothes your lines.
That’s great if you want that. But sometimes we want illustrator to respect the jaggedness of your drawing, and not smooth things out so much (like if you are drawing a box by hand).
To reduce the smoothness, double click the brush and drop the fidelity down to 0 (or as low as you can go).
This will remove the smoothness, and make your lines less smooth, and more jagged.