One of the major benefits of the overlay2 filesystem for Docker is that you no longer need to have a separate storage volume for your Docker containers, images and volumes. This means that you don’t need to try to decide how much free space you need in the root FS as opposed to how much you need for containers; you can just create one big volume to fill up the disk.
Since overlay2 is fairly new, though, we’ve chosen to install a separate Docker volume by default on Fedora 26 Atomic Host. This lets users switch back to devicemapper if they run into some kind of issue with overlay2. However, if you’re installing a new dev system, you might want the ease-of-management of having one big volume.