Why is defining the light source important in value studies?

Value isn't just important to drawing: it's what allows us to see form. Our entire visual experience and perception of the world depends on light.  Whenever we see anything, it means that what we are looking at is being illuminated by a light source. As an object turns away from the light source, less light can reach it, so it gradually darkens. Eventually  the object falls into shadow where light can no longer reach it. This incremental darkening of an object turning away from light creates a range of values.

Explanation: