Ok, for example.
Thick lun with enabled snapshots:
You have Volume 200GB, free 200GB.
You create thick LUN with 100GB Size. Free size at volume: 100GB, because thick LUN reserve size.
You map LUN to windows server and put 50GB file on it. Now you have 50GB free at Volume, because 100GB LUN + 50GB Snapshot.
You delete 50GB file from system. Now you have 100GB free space at Windows server, but you still have 50GB free at your NetApp volume.
You delete Snapshot. Now you have 100GB free at Volume.
Thin lun with enabled snapshots:
You have Volume 200GB, free 200GB.
You create thin LUN with 100GB Size. Free size at volume: 200GB, because thin LUN does not reserve space.
You map LUN to windows server and put 50GB file on it. Now you have 100GB free at Volume, because 50GB used space at LUN + 50GB Snapshot.
You delete 50GB file from system. Now you have 100GB free space at Windows server, but you still have 100GB free at your NetApp volume, because deleting files from thin LUN does not free up your storage.
You delete Snapshot. Now you have 150GB free at Volume.
But, as i know, there is some SCSI technologies, that can help you to free some disk space from storage, using thin LUN.