Grammar, numerical errors, wrong archive uploads... and any human error (or even machine) we could do and would love to correct. If you can delete and repost the correct version it improves the experience overall:

- For the original poster: can keep it's feed clean.

- The client: improves the user experience. Also makes their business more law-ready, since some legislations consider the "Right to be forgotten" a thing, so they would comply with it.

- The relay: same as for the client, and they also avoid storing unnecessary duplicated data that costs money.

- The reader: avoids having to read notes that have errors, or successive notes clarifying the corrections.

At the end is more about de-indexing than actually deleting, because some relays could not honour the request, and that's great, but I believe most actors will benefit from it.