Lines can be delicate or aggressive, directional or meandering, representational or abstract. Lines do much more than delineate a shape. They have rhythm. They have motion. They can fill space, enclose space, or create negative space.
Vertical lines draw our eyes upward like tall trees and make us feel uplifted. A horizontal line feels calm and at rest. Wavy and curly lines imply whimsy. A diagonal line is the most energetic and suggests movement. If you’re in business, there is nothing you want to see more than a graph line moving upward.
Sometimes a row of objects creates the impression of a line that is not actually there. Where two fields of color meet also implies a line. Many lines together can form a pattern, for example a crosshatch.
You can do so much more with lines than outline. In fact, one of my rules is avoid outlining shapes, especially type! Outlines add unnecessary complexity. If you feel you need to outline something, it usually means there is a lack of contrast. Fix the underlying problem. And have fun with lines!