The manual entry for --show-session-key talks about being forced to reveal the
content of a message by a government or similar power. However, there is another
use case:

If someone sends you an abusive or offensive message over some messaging system,
if it is unencrypted you just have to flag it to the administrators and they can
see that it is offensive and take appropriate action. However, if the message is
encrypted you need some way of proving that the ciphertext sent to you
corresponds to an inappropriate message. '--show-session-key' allows you to do
this without revealing the content of other messages.

A possible phrasing for the documentation:

You can also use this option if you receive an encrypted message which is
abusive or offensive, to prove to the administrators of the messaging system
that the ciphertext transmitted corresponds to an inappropriate plaintext so
they can take action against the offending user.