It gives you one image per configuration, instead of one build machine, or one stored VM. It makes it easier for a single machine to iterate over dozens or hundreds of build environments with a minimum of overhead.
For example, one Ubuntu machine with Docker can create build environments for every Linux based distro (and version thereof) that exists, and iterate through all of them when it comes time to build. The setup is not trivial, though it is easier than creating a similar environment with VMs or physical machines.
