Docker a low overhead way to run virtual machines on your local box or in the cloud. Although they're not strictly distinct machines, nor do they need to boot an OS, they give you many of those benefits.
Docker can encapsulate legacy applications, allowing you to deploy them to servers that might not otherwise be easy to setup with older packages & software versions.
Docker can be used to build test boxes, during your deploy process to facilitate continuous integration testing.
Docker can be used to provision boxes in the cloud, and with swarm you can orchestrate clusters too.
