The original (custom) message cannot be found by the sender - the messages section of the proof does not contain the original message added upon creation. I think there used to be a checkbox to at least send that to the creator, (now) it's not there (anymore?).
For audit purposes and in order to have a reference for the author who should be able to show evidence of what he/she wrote and push back or confirm potential blames.
That's why this should be there as an option minimum, but the best would be to record that message and make it accessible for them or all involved parties.