When you want to show how A, B and C add up to the total of ABC.

For example if “you would like to show …”

… the market for sportcars is split into different car brands, you can use a 100% barchart.

… to what subpage of your website the viewers of your homepage go to, you can use a sankey diagram.

… the percentage of energy left in your phone, you can use an image fill.