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).

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.

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".

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.