The character of line can shape an image entirely. A line is not just a line. There's something fundamental about the line. It is a foundation of structure, or at least our understanding of structure. A set of intersecting lines is used to represent not just contour but also volume.
Edge is a basic fundamental of the visual language that helps representational images hold together.
Line quality must be carefully controlled to ensure that all the elements in an image appear to share the same reality. The former is a thing in and of itself, the line as object. The latter is an optical phenomenon. I'm primarily concerned with here. Line, when it is not the figure itself, helps separate figure from ground or object from environment.