This hook allow you to manipulate the email template without editing it in the settings. as an example, it may prove useful on Multisite installs to add site wide text to the end of all emails.

function my_s2_template($template) {
    $template .= 'Some additional text I would like at the end';
    return $template;
}
add_filter('s2_email_template', 'my_s2_template');