Disk failure.

Accidental file deletion.

Random file corruption.

Complete machine destruction, say by fire, including destruction of any on-site backups.

Archives of the whole system, backed up onto permanent, off-site media. This provides protection against all of the problems listed above, but is slow and inconvenient to restore from. Copies of the backups can be stored on site or online, but there will still be inconveniences in restoring files, especially for non-privileged users.

Filesystem snapshots, which are really only helpful in the accidental file deletion scenario, but can be very helpful in that case, as well as quick and easy to deal with.

Copies of whole file systems or disks which can be created with a periodic net/rsync of the whole machine. This is generally most useful in networks with unique requirements. For general protection against disk failure, this is usually inferior to RAID. For restoring accidentally deleted files, it can be comparable to UFS snapshots.

RAID, which minimizes or avoids downtime when a disk fails at the expense of having to deal with disk failures more often, because there are more disks, albeit at a much lower urgency.

Checking fingerprints of files using mtree(8). Although this is not a backup, this technique indicates when one needs to resort to backups. This is particularly important for offline backups, and should be checked periodically.