The Corresponding Secretary is the official voice of the organization. If the President is the soul, the Second Vice President is the brain, and the Sergeant at Arms is the hands of the organization, the Corresponding Secretary is its gaping, toothy maw. The Bylaws describe the CorSec by saying:
The Corresponding Secretary shall have charge of all correspondence pertinent to the organization other than the correspondence reserved for the Recording Secretary and Treasurer. The Corresponding Secretary shall be the Editor of the organization Newsletter.
That is to say, the CorSec makes all official announcements, conducts all official correspondence on behalf of the organization (including reading and answering the email), reserves space for organizational meetings and events, and picks up the physical mail at the mailbox in the UC. In addition, the CorSec is the editor of Pravda?, that most exemplary of publications.
At times, the CorSec has also announced upcoming events, though this task is often best conducted by the Second Vice President. Also, the CorSec has historically been in charge of updating the website, though that task is large enough that an ad-hoc position would probably be appropriate.