Because of our human ability to understand relationships quickly based on size, position and other spatial attributes, the eye can summarize what might otherwise require thousands of numbers to convey.
I have heard many cartographers say that if a picture is worth a thousand words, a map is worth a thousand pictures. This description of our human ability hits at the core of the power of data visualization. There are many great books and blogs dedicated to the subject of data visualization, and the tips provided apply to data visualization, cartography, or software development for that matter. It all comes back to finding the most efficient way in which our minds turn data into information. This equates to better decisions. This is the goal of business intelligence (BI) software and why BI software should not compartmentalize, but emphasize the visualization of spatial data.