Many site owners use 'header.php' and 'footer.php' include files so that only a single file has to be changed to alter the header/footer across an entire website.  These files are generally included on a webpage with statements like:

    require_once "includes/header.php";

This plugin allows you to specify a path to those various include files to wrap each MyBB page with consistent branding without modifying any MyBB template files.  Any changes you make to your header/footer are instantly reflected in MyBB.