Mailing List Subscriptions module allows you to put a form allowing to subscribe to and/or unsubscribe from a mailing list anywhere in your webpage. When a visitor fills in the form, this module generates a subscribe or unsubscribe e-mail message and sends it off to a defined robot address. If the mailing list requires more than just the initial (un)subscribe message to process user's request, the user themselves will have to proceed from this point. Most likely, in such case they will receive an email with easy instructions to follow. Basically, this module is a convenient way to avoid bothering users with "sending an empty message to address xxx" or introducing them to those ugly pages that Mailman generates. :)