Why is it hard to understand? Because your average user will not "wireshark their own cables", and those who do will use different methods of secure communication. You'd be surprised how many users are unable to bypass various extremely simple restrictions - and this is the target market for these features.
On top of that self deleting email will make it also a bit harder to prove that the screenshot is real - unless you get a court order to get the senders outbox folder from google and the data still exists there.
How do you know this? Enterprises have archival/deletion policies automatically applied and emails you forward to outside parties are often tracked in legal hold platforms.
It's not unusual for whole sales organizations in particular industries have all emails archived under legal hold requirements (whether sent or received.) Another set of organizations will purge all of the same email older than than a certain date.
None of these measures are particularly hard to defeat for a bright user, but many of the risks at the company-level are mitigated. This is mostly to solve challenges with regulation or company certification, but it can make a real difference when someone is handling a privacy incident.
On top of that self deleting email will make it also a bit harder to prove that the screenshot is real - unless you get a court order to get the senders outbox folder from google and the data still exists there.