D3 layouts are modules that transform data into drawing rules. The simplest layout might only transform an array of objects into coordinates, like a scale.

However, we usually use layouts for more complex visualizations, such as drawing a force-directed graph or a tree, for instance. In these cases, layouts help us to separate calculating coordinates from putting pixels on the screen. This not only makes our code cleaner, but it also lets us reuse the same layouts for vastly different visualizations.