vSphere HA is a cluster-wide setting. This means that once it's enabled, it protects all virtual machines that are a part of the cluster.

In most cases, there are more important virtual machines running along with less critical VMs on the same cluster, and it's desirable that more important and critical VMs are restarted first. Also, depending on cluster resources available, vCenter may not be able to restart all virtual machines on other hosts, so it becomes even more important to make sure the most important servers will be backed up first.

Each environment has critical virtual machines, which in case of host failure need to be restarted before other VMs. A good example is domain controllers, which in most cases need to be up before other servers become available. Another example is database servers, which may be required online by applications and web servers. In such cases, the ability to configure VM restart priority can be very useful.