Good for blocking big chunks of value. With very light stroke (middle image), you can see some texture and underlaying image is not destroyed/blured. With medium pressure you will blur your underlaying image and apply some paint (right). With hard pressure you basically paint with full opacity.
To make this brush edges hard, when using big brush size, I had to set spacing to low fixed value so that you get nice sharp edges. That is why be warned: when using small brush size, you may see ugly stepping effect. Basically this problem is related to my post here:http://paintstormstudio.com/forum/viewtopic.php?f=5&t=2824