You can stretch a cluster in the management domain or in an NSX for vSphere VI workload domain across two availability zones. You cannot stretch a cluster in an NSX-T VI workload domain.
You may want to stretch a cluster for the following reasons.
You can perform a planned maintenance on an availability zone without any downtime and then migrate the applications after the maintenance is completed.
Stretching a cluster automatically initiates VM restart and recovery, and has a low recovery time objective for the majority of unplanned failures.
With a stretched cluster, you can prevent service outages before an impending disaster such as a hurricane or rising flood levels.
Once a cluster is stretched, you cannot un-stretch it.