Pie charts aren't perfect but are useful when applied correctly.

Two great things about pie charts:

1. From the shape you can tell you are looking portions of a whole. For example, a pie chart of market shares shows all businesses in the category.

2. You can easily guess the proportion of the segments relative to each other. For example, in this chart http://www.liv.ac.uk/~cll/lskills/WN/pie%20chart1.gif you can tell that football and rugby make up half the total and cricket, tennis, athletics, and other make up the other half.