Thus, participants are pooled under 1 time constrained key exchange, participants sections origins are signed/time-stamped, and the ciphertext expires on a schedule.
Why? so at some future time side-channel key exfiltration can't be used to data mine the entire history of past tapped conversations/ciphertext. Even if a private signing key is stolen, it is difficult to rebuild the lost ephemeral key session.
Have a nice day. =3
Depends what you do, and if your organization has a persistent clown infestation. Some big business folks still use email as a human to API interface.
Running out-of-band public keychains is not something most people will ever need. =3