Clear presentation of data using graphics are critical to how fast people can understand the information and how comfortable they are in interpreting the information.

Great article using the Edward Tufte visualization of the data from the '86 Space Shuttle crash. The engineers were very concerned about the temperature on the day of the launch, which they felt heightened the risk of the O-rings being damaged.

The engineers presented all the data to the decision makers through multiple reports and with the data spread out on many different pages. Of course it was hard to put all the data together and understand the severity of the issue.

The actual fax of one page of the data to decision makers

As you cab see, not really something that shows the severity of the issue.

Edward Tufte's visualization of the severity for the launch

The graph shows 2 things.

The dots are previous launches and the severity of damage to the O-rings. As the chart clearly shows the colder the temperature, the higher the risk for damage
The Red X marks the temperature on the day of the launch in question. Because the temperature is used on the axis and includes the launch day, it shows how far out of the normal ranges this launch was and since the damage increases as the temperature decreases, this shows the severity in a way that would have more then likely stopped the launch on that fateful day.

Now most of us won't be presenting data that will save lives like the example above, however it really shows how a good piece of visualization shows outliers and gives the decision makers easy to understand data to make better decisions without having to go through pages of data.