When used with a SAN, ESXi can benefit from multiple vSphere features, including Storage vMotion, Distributed Resource Scheduler (DRS), High Availability, and so on.

Using ESXi with a SAN is effective for the following tasks:

Storage consolidation and simplification of storage layout
If you are working with multiple hosts, and each host is running multiple virtual machines, the storage on the hosts is no longer sufficient. You might need to use external storage. The SAN can provide a simple system architecture and other benefits.
Maintenance with zero downtime
When performing ESXi host or infrastructure maintenance, use vMotion to migrate virtual machines to other host. If shared storage is on the SAN, you can perform maintenance without interruptions to the users of the virtual machines. Virtual machine working processes continue throughout a migration.
Load balancing
You can add a host to a DRS cluster, and the host's resources become part of the cluster's resources. The distribution and use of CPU and memory resources for all hosts and virtual machines in the cluster are continuously monitored. DRS compares these metrics to an ideal resource use. The ideal use considers the attributes of the cluster's resource pools and virtual machines, the current demand, and the imbalance target. If needed, DRS performs or recommends virtual machine migrations.
Disaster recovery
You can use VMware High Availability to configure multiple ESXi hosts as a cluster. The cluster provides rapid recovery from outages and cost-effective high availability for applications running in virtual machines.
Simplified array migrations and storage upgrades
When you purchase new storage systems, use Storage vMotion to perform live migrations of virtual machines from existing storage to their new destinations. You can perform the migrations without interruptions of the virtual machines.