The snapshot reserve is really there for filesystem which are exposed to end users, who may not be aware of the behaviour of enterprise storage.
Normally, if you use snapshots, then if a user deletes a file, it does not actually free any space on the storage system (assuming the file is in a snapshot). The end user may find that confusing ("I just deleted that big file but my space used didn't go down"). So by hiding some of the volume space from the user in the snapshot reservation, you make the user think that when the delete a file that space really is freed up.
NetApp best practise for volumes which aren't traditional NAS mounts/shares (e.g. volumes which contain LUNs) is to set the snapshot reservation to zero. You might also want to do that for volumes which end users don't directly see, such as backup destinations. And of course if you don't take snapshots then you don't need a snapshot reservation.
Setting a snapshot reservation will also prevent an end user from causing snapshot failures in the volume by filling it up, but only provided the snapshot reservation is not full.
Jeremy