The man page for Terminal says 
--role=role
           Sets the window role of the last-specified window to role. Applies
           to only one window and can be specified once for each window you
           create from the command line. It is mostly used for session
           management inside Terminal.

Can someone (who is in the know) describe 
(a) what its use in session managemant is, and
(b) what is it used for aside from session management?

Thanks.