Our minds are always able to comprehend pictures much faster than anything else.
Data Visualization can be used for two purposes :
1- Illustration & Presentation
Minard's Visualization Of Napoleon's 1812 March which tried to illustrate the cartographic depiction of numerical data on a map of Napoleon's disastrous losses suffered during the Russian campaign with different dimensions.
Drawing some points and trying to infer a relationship or at least an understanding of the data before any mathematical treatment.