Donut Keynote chart is essentially the same pie chart, or cake chart, with the center cut out.
At the same time, donut charts have some advantage over pie charts, which are sometimes criticized for emphasizing the relative sizes of parts in relation to each other and the chart as a whole, without indicating changes in general for comparison with other pie charts.
Donut Keynote charts provide an answer to this problem in part by reducing the focus on specific areas, allowing the observer to focus more on changes in overall values. Here your attention will be focused on the length of the arcs, and not on the comparison of proportions between the pieces of cake.
Donut charts for Keynote also take up less space than pie charts, allowing you to display additional information within an unfilled circle.