It demarcates beginning and end, perhaps not always the true end of something, but certainly the end of our ability to perceive it. They are not fixed in space. They move as we move. Picasso claimed so. Monet said no.
Lines are mercurial in nature, they change based on the context in which they are seen.
If contrast is weak, then lines are weak. If contrast is strong, then lines are strong. The stronger the difference, the stronger the line. Op artists capitalize on this. Part of what we see in their brightly colored canvases is not painted.