Although instances are virtual, not physical, they are hosted on physical servers and use resources while they are up.
So it is important to stop the machines when they do not need to be running. And remove those that are no longer useful. Thus, you free up allocated but unused resources.
Taking a snapshot of instance in order to restore or migrate to another OpenStack project.
$ openstack server image create <instance> --name myInstanceSnapshot
If instance is up, data are not guaranteed to be flushed to disk.
Stop an instance when does not need to run continuously.
$ openstack server stop <instance>
Delete a no longer usefull instance to free up unused allocated resources.
$ openstack server delete <instance>
Furthermore, it’s an eco-responsible approach !