You can also use it on a single drive. You need to set copies=n for a ZFS dataset. If one copy gets corrupted a zpool scrub fixes this problem on a single drive. However HDDs are usually dying fast after signalling bad blocks to the OS so it's not a good strategy to avoid data loss.

Besides that the metadata is still replicated in a single-drive setup. So if you loose data you'll at least know exactly where the error was. This is still a benefit over ext4.