In particular, it can be used to enable ACLs, or to remove the 92% storage cap from a nearly-full filesystem. A sysadmin with a "full" filesystem and panicky users can reclaim this missing 8% storage space temporarily while s/he figures out how to permanently add storage space to the system:
# tunefs -m 0 /mnt/myvolume
Note that while this DOES buy the sysadmin some time - 20GB worth of time, on a 250GB partition - it does so at the expense of performance and an increased risk of crashing applications or the system itself, depending on what's stored on that partition. The wise sysadmin will make certain to restore the storage cap to its default value once s/he has made whatever permanent changes (adding new drives, deleting gigabytes upon gigabytes of dead files, moving stuff to different partitions, etc) were necessary.