vSphere volume grow allows dynamic expansion of a VMFS volume without disrupting the virtual machines running on that datastore.
The underlying LUN on the storage array should be extended first which is then made available to the ESX host. There are two possibilities here - Extend the same LUN in the storage or create a new LUN and add it as an extent. Extending the same LUN may or may not improve performance based on how the LUN is laid out on the spindles. This method will have less management complexity and safer as there is only one LUN to protect
Creating a new LUN to add as an extent will improve performance if they are laid out on different spindles on the array. This adds more LUNs to manage and the datastore will be inaccessible even if one LUN is corrupted. So there are more LUNs to protect & manage.
With VI3, creating a new LUN in the array and adding it as an extent will work without the need to turn off the VMs. Extending the same LUN will not work when the VMs running on the VMFS volumes are powered on. A simple workaround is to do a storage vMotion to extend the same LUN
With vSphere, there is no need to turn off the VMs to extend the same LUN in the storage array.