Sometimes you just want a raw count of the number of work items you are looking at.
Use the Open vs. Closed chart for a quick overview of your team's progress in purely numerical terms. A glance at this chart can suggest some broad generalizations, which you'll then want to test by examining your other charts and your planning folder contents more closely.
The Open vs. Closed chart is good for some basic summary information. You may want to share this chart with executive sponsors who need to know how things are going in quantitive terms, but don't need a lot of detail about the types of work being addressed.