This is a great list of links on Data Visualization.
Visualization is a different way of looking at large amount of data. You will notice the difference when the same data is in a tabular form and presented next to the pictures. Many visualization techniques let you navigate (or hyper navigate) through data. That is one of their strengths.
Visualization adds a different dimension for manipulating data, especially if it is interactive. You will notice things (like clustering) that may not be as easy to find in the tabular form (especially when the data is hundreds or thousands of rows).