Prior to vSphere 5.1 if you want to do both change host and data store then the VM needs to be Powered OFF, So the sVmotion was the feature used to migrate the VM's to different datastore without downtime.
Even in vSphere 5.1 we have to use the webclient to get benefited of change host and data store. If the vSphere Client is used we can only do Change Host or Change Datastore online.
