For example, I have a field called "Heat Level" where the users can set the item to be Red, Yellow, or Green. (It is similar to a priority, but not exactly the same.) When I create a pie chart, the pie pieces could be any random color. Our users complain about the pie pieces not matching. In other words, they want the pie piece called "Red" to be "red". They don't want the piece called "Red" to show in blue (or other color). The same happens with bar graphs.
As another example, sometimes the bar chart will have red and orange next to each other, making it very hard to read. If I could control the colors, I could make sure similar colors were not used, or at least make sure they were not next to each other.