While reviewing a signed document depending on security you can or can not make comments or highlight areas. Makes it difficult to review or mark text/images for follow up. This would allow you to make notes and provide comments or analysis to support signing or returning a document for editing. Especially documents that have multiple reviewers or signers this allows the analyst to provide supporting info on the document itself. Ideally this would be like an overlay and not part of the original file so the signature would not lock these into history and the notes pages could be sent with the file or kept separate (close hold)