When a certain segment on which you are reporting is important, you might want to know more about it and how it relates to other dimension. This is often called breaking down a dimension by another dimension. A breakdown is the action of integrating two or more Analytics variables.
You can break down dimensions by other dimensions to analyze and compare nearly anything. For example, you can view trends across other continuous dimensions like Page Depth. Then you could visualize a metric like Revenue Participation to see how revenue is influenced as a visitor travels deeper into your site.