Snapshots make saving/restoring state near instant, assuming no hardware failure. With zfs they also allow incremental replication.

A lot of different uses for that, from mitigating non hardware related issues like user error or malware in a way which allows much faster recovery than backup to simple experimentation.

Example unrelated to zfs, but still snapshots - at work we use microsoft shadow copy to give users ability to restore old versions of files on network shares. Being able to pull hour old version of file if you accidentally deleted or overwritten it is very handy, and unlike backup this is instant, easily accessible and does not require massive amount of storage.