Sometimes the primary analytical task at hand is to understand what happened between two points in time. For example, did company expenses increase, decrease or stay unchanged between this year and two years ago?
There are different ways to present change between two points in time. Slopegraphs, Range Bar Graphs and Deviation Graphs (both in magnitude and percentage) are three different approaches that may be used to show that change. To each its advantages.
Stephen Few wrote a thorough paper explaining these three different approaches along with rating the strengths of each individual approach.
The top chart below is an example from Edward Tufte. The same dataset is shown on the bottom below using different approaches.