--show-session-key does not appear to be supported with --encrypt. This is useful for caching the session key, e.g., when encrypting a draft that is saved on the imap server, but should be accessible without having to enter a passphrase or inserting a smartcard.

Consider:

$ echo foo |  gpg -a --show-session-key -e -r neal@walfield.org

gpg: NOTE: THIS IS A DEVELOPMENT VERSION!
gpg: It is only intended for test purposes and should NOT be
gpg: used in a production environment or with production keys!
-----BEGIN PGP MESSAGE-----

hQEMA8K4GQVsZSWYAQf/fuNYRXtS2xcGhfwJgsNeuQMenmSY8ossgFtIEh8THf23
MUqV+f29/QFbALDTqCwD33eHoV/SxQYrc71aREqbaOeirg4jQflHrSwRKs0Az6dd
RMoPc+zLLC0GHrOk5jre/UVIl1mzw2gj4JjywGUSIFjZ1lVIG5wq7HBChHUUZV1a
GHHwVyWaagqPIqWrLD5ExEK5DzQ2V5g6umNauMabcx23XpyNV0+MovKiUUXFXxsb
i97aF8ACRmqDo8ouDLhLYrrE0MUqi6I/zdpbI0rixlWIxGMXqPLs4wR1GHshiMe2
KksR4efveeWjPmxsvdmsLflFRlbYxaXgBXDNqgAvDtI5AbpuQOE3gQUsj5v+JkBW
OizyusZu++mMAg9KqHcjdewqMhhjExq6b8idC9NF1xzyhlBcvHHh5W1k

zW8/

-----END PGP MESSAGE-----
us@grit:~$ echo foo | gpg --status-fd 1 -a --show-session-key -e -r neal@walfield.org
gpg: NOTE: THIS IS A DEVELOPMENT VERSION!
gpg: It is only intended for test purposes and should NOT be
gpg: used in a production environment or with production keys!
[GNUPG:] KEY_CONSIDERED 11C294DF1D6C9698FEFE231D3BF609C68BAFCDBD 0
[GNUPG:] KEY_CONSIDERED 11C294DF1D6C9698FEFE231D3BF609C68BAFCDBD 0
[GNUPG:] KEY_CONSIDERED 520054A53C19CBB2E7F5639687234295786B0BAD 0
[GNUPG:] KEY_CONSIDERED 8F17777118A33DDA9BA48E62AACB3243630052D9 0
[GNUPG:] KEY_CONSIDERED 8F17777118A33DDA9BA48E62AACB3243630052D9 0
[GNUPG:] KEY_CONSIDERED 8F17777118A33DDA9BA48E62AACB3243630052D9 0
[GNUPG:] KEY_CONSIDERED 8F17777118A33DDA9BA48E62AACB3243630052D9 0
[GNUPG:] ENCRYPTION_COMPLIANCE_MODE 23
[GNUPG:] BEGIN_ENCRYPTION 2 9
-----BEGIN PGP MESSAGE-----

hQEMA8K4GQVsZSWYAQf7BwA7VsBows3wMeDd7mtJuRWQkzmQtw5dkTtjla9t1pu0
bA6KvLmdW3oz2dyKBHiK0gCQlpMw4cBkRDTW5x/c/sV+WFew3YzZRESWmsNXQUOM
6aNoJ3Qj/gxQ+fTqUKXipkHWDvqqT53/eu6jguQ9DC49j1yZPtn0Gl+nxlDp7HFe
FN+pG0++Z63EZsftZ+Akemi8TVlGZE3lY/KB1YYdZF/Guh/6MtNII7nYNUqL8So/
Gk22Mmrt7luMSmybknhT7NiI7tXcfZHyAa0wTjXukiEyvLiPtgcJ7qhLDDm/WFPw
V3dqimfJ22PCMflh/orsgb5nrV83mv0bTpxa1mBI9NI5AfYeaVceRncGuxqf6A2U
+IxAzjKDhv83WrQKJdNIc0UVaFdu8v9ZxtI+yK033VMi94iDiRHii2j8

kXUd

-----END PGP MESSAGE-----
[GNUPG:] END_ENCRYPTION