Setting fractional reserve on a volume to zero and managing your free space manually, rather than having Data ONTAP manage it for you, gives you more control over space utilization.
You can maximize your space utilization by exercising more control over how much space is allocated for overwrites. The cost is extra management time and the risk of encountering out-of-space errors on a space-reserved file or LUN.
For example, setting fractional reserve to zero might be helpful for a Qtree SnapMirror destination of a space-reserved LUN. (For Volume SnapMirror, the fractional reserve of the target is always the same as the source.) Depending on the rate of change (ROC) of your data, how much free space is available in the volume, and whether you are taking snapshots of the volume containing the target, it might make sense to reduce the fractional reserve to zero for the target.