Convex hulls can be useful for grouping points on a scatterplot. For example they can be used to group countries by continents if they are close to each other, or, in this case, to group different flower species and reinforce the effect of the fill.
Implemented with d3.polygonHull.
If you use ggplot2 the package ggalt implements geom_encircle, a geometry that achieves the same result.
Some examples:
Trump Sounds a Different Tone in First Address to Congress, The New York Times.
The Word Choices That Explain Why Jane Austen Endures, The New York Times.
Climate change will affect developing countries more than rich ones, The Economist.