Purpose:
To determine where values change when light moves over an uneven surface by using a range of values to paint the different values in a realistic, monochromatic self-portrait;
To effectively communicate an image that looks like you through careful examination of the values and structure of your face.
I think that many different skills are required to paint a good self portrait value painting. One example of an important skill is shading. Shading is very important because it is needed to create value and make the face more Three-Dimensional. Another skill that is needed for self portrait value paintings is scale, you need to draw objects to scale or the painting won't look like the object at all no matter how good of shading is done.