[fragmede]

Not OP but if party A is harassing party B (read: "sending unsolicited dick pics"), I could imagine circumstances under which the sysadmin deletes messages that party B has received. (After HR and lawyers all around have been looped in by all parties, and copies of the messages have been forwards to the lawyers.

Also consider that the first amendment isn't absolute and there is certain material that is highly unsavory, eg child pornography, that party B doesn't even want the potential of possessing.

There are certainly nefarious usages for that level of access as well, but I can imagine legitimate usage exists as well.