As far as i know here is why you use VMware ESX Clusters:
- if one of the physical hosts goes down, the other physical host starts up the virtual guests machines that the original virtual host was running (VM HA).
- if one physical host is over utilized by a virtual guest, that virtual guest is moved to the other physical host (VMware DRS, discussed in the resource pool section, below).
Both of these features use VMotion to move these virtual guests from one system to another.
Best regards
Yours, Oscar