ZFS writes a checksum when it writes data. So you can tell whether your data is corrupted, even if you don't have the redundancy to restore the corrupted data.

Error detection can always be used, but error correction may or may not be available (it depends on the type of block). Metadata blocks are redundant even on a single drive pool; so if you just have a partial failure (e.g. overwrite a metadata block) it might be able to correct the block using another redundant copy on the same drive. Data blocks will require a redundant pool configuration, though, as these are not store redundantly by default (e.g. multiple drives in a raidz or mirror).

It only works in the sense that if it finds an error it could recover when it is a multi-drive RAID, but a scrub can verify that the data on disk is still whole and matches what it expects.

Not only that, but a scrub can have ZFS tell you exactly what files are now "damaged".

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.