In a nutshell, lines are like markers to direct your visitors. In some cases they will act as stop signs. In others, they will take the visitor by the hand and lead them where you want them to go. But no matter what action lines cause, they also have a secondary role. Without dividers and direction, our eyes will wander all over the page.
Lines can help elements feel more solid on the page as well.
As an example, look at the samples to the left. In the upper sample, the graphic is lined up neatly with the text , and forms a spot for the eye to rest. The second sample is identical, with the addition of a line. In the first sample, our eye is stopped and we will then move naturally to the headline. But in the second, we stop and focus in on the headline in one movement.