I believe this was created and presented as simple (contrast to easy) demo to demonstrate BTRFS as a possible backend for images. Docker currently uses AUFS, and was designed to support more fs backends over time. It also demoed a potential path for docker to become more extensible over time.

A github issue was created to incorporate BTRFS: https://github.com/dotcloud/docker/issues/443

Dotcloud blog post on dockerlite: http://blog.docker.io/2013/05/btrfs-support-for-docker/

Source: I saw the author's presentation at the docker meetup held at dotcloud a couple months ago.