Emphasise variations above or below a fixed reference point. Typically, the reference point is zero, but it can also be a target or a long-term average. These types of charts are also often used to show sentiment (positive/neutral/negative).
Smith, Alan. How Charts Work (p. 155). Pearson Education. Kindle Edition.
When considering deviation, we naturally form an association with positive and negative figures, in either absolute (e.g., profit/loss) or relative (sales figures above/below target) terms.
Smith, Alan. How Charts Work (p. 161). Pearson Education. Kindle Edition.