Need for space reclamation will be needed of there are following situation.
1. If there is a thin lun from the storage and you have deleted few vms or did a storage vmotion.
2. Esxi host does not do auto spce reclamation because it is an over head for hostd and vmkernel.
3 if you have VAAI feature then it can happen automatically because host will offload the task to the storage.