docker-machine is dangerous. I wouldn't recommend it for managing production servers as it requires passwordless sudo and makes it very easy to damage your Docker installation. I managed to completely remove all containers an images from a server, not realizing the command I ran was not merely connecting to the server, but initializing it from scratch.
If you want to control multiple Docker daemons from single CLI try Docker Contexts.
Edit:
docker-machine's purpose is provisioning and managing machines with Docker daemon.
It can be used both with local VM's and with various cloud providers. With a single command it can create and start a VM, then install and configure Docker on that new VM (including generating TLS certificates).
It can create an entire Docker Swarm cluster.
It can also install Docker on a physical machine, given SSH access with passwordless sudo (that is what generic driver you tried to use is for).
Once a machine is fully provisioned with Docker it also can set environment variables that configure Docker CLI to send commands to a remote Docker daemon installed on that machine - see here for details.
Finally, one can also add machines with Docker manually configured by not using any driver - as described here. The only purpose of that is to allow for a unified workflow when switching between various remote machines.
However, as I stated before docker-machine is dangerous - it can also remove existing VMs and in case of physical machines reprovsion them, thereby removing all existing images, containers, etc. A simple mistake can wipe a server clean. Not to mention it requires both key-based SSH and passwordless sudo, so if an unauthorized person gets their hands on an SSH key for a production server, then that's it - they have full root access to everything.
It is possible to use docker-machine with preexisting Docker installations safely - you need to add them without using any driver as described here. In this scenario, however, most docker-machine commands won't work, so the only benefit is easy generation of those environment variables for Docker CLI I mentioned before.
Docker Contexts are a new way of telling Docker CLI which Docker daemon it's supposed to communicate with. They essentially are meant to replace all those environment variables docker-machine generates.
Since Docker CLI only communicates with Docker daemon, there is no risk of accidentally deleting a VM or reprovisioning already configured physical machine. And since they are a part of Docker CLI, there is no need to install additional software.
On the other hand, Docker contexts cannot be used to create or provision new machines - one needs to either do that manually or use some other mechanism or tool (like Vagrant or some kind of template provided by the cloud provider).
So if you really need a tool that'll let you easily create, provision and remove docker-enabled machines then use docker-machine. If, however, all you wan is to have a list of all your Docker-enabled machines in one place and a way to easily set up which one your local Docker CLI is supposed to talk to, Docker Contexts are a much safer alternative.
