Attorney client work privilege is a legal rule that means that any material that is compiled by an attorney or on behalf of them during a case for a client is confidential in the same way as attorney client privilege. This means that the other side cannot ask for any material to be brought in as evidence that would be derived from a client speaking openly to their attorney and the attorney's office making notes of such words.
Without this natural follow on to attorney client privilege, it would create a situation where the attorney would not be able to take notes. That would clearly not be in the best interest of their client.