Remember when virtual machines came out? That technology gave us tools to create a 'layered' effect for compute related resources on a single, managed machine. Microservice technologies, like Docker containers, are a logical extension of that layering into the process space on the machine, and in many cases multiple virtual machines running on a single machine.
Joyent actually extends this and allows you to treat an entire datacenter as your docker host, similar to how you can use a product like VMware to cluster ESX hosts together.
It's all about the layers, man.