Yes, and no.
You can use ZFS snapshots as a poor-man's backup solution, in that you will have old versions of files accessible. However, using the same set of disks for "live" data and "backup" data is just asking for trouble in the long-run. This is what I do at home (3-disks in a raidz1 configuration, using daily ZFS snapshots to keep 30-60 days of "backups" around).
However, you can also use snapshots as a staging ground for creating off-system backups (copy the contents of the snapshot off to another server), and you can also create a backup server that uses snapshots to provide access to historical data (this is what we do at work; rsync data off remote server to ZFS, create snapshot, repeat daily).
It all depends on what you need, and what hardware you have available.
UFS snapshots are a little more tricky, but are still usable in roughly the same ways.