This is the ramdisk element.
Almost any user building a ramdisk will want to include this in their build, as it triggers many of the vital functionality from the basic diskimage-builder libraries (such as init script aggregation, busybox population, etc).
An example of when one might want to use this toolchain to build a ramdisk would be the initial deployment of baremetal nodes in a TripleO setup. Various tools and scripts need to be injected into a ramdisk that will fetch and apply a machine image to local disks. That tooling/scripting customisation can be easily applied in a repeatable and automatable way, using this element.
NOTE: ramdisks require 1GB minimum memory on the machines they are booting.
See the top-level README.md of the project, for more information about the mechanisms available to a ramdisk element.