These are just brush strokes to help break up the boring shadow shape. Look at the light side of the hat, they do the same thing with a blocky brush and opacity/pen pressure.
The clothing has some cool brush texture that follows the folds too. >>5626376
With this one, there are small plane changes that the artist exaggerated to show form. Instead of making smooth transitions, they chose to keep the plane changes sharp. Again, using brush strokes and shapes to break up boring, smooth plane changes.
If in doubt, just do studies of the art you like to get a better understanding. It will all come with time and practice.