Right now it's not possible to create navigation menus in the global site. Having this ability would allow portal admins to set common navigation menus that would show up in each and every site as read only. Right now to create common footer navigation across all sites, it's not really possible and if it is possible it doesn't seem intuitive enough for normal people to set up.
This would allow better dissemination of footer or header links across all sites of a portal, without needing to dig into a theme template file. It's possible to achieve right now but usually only by developers with deep Liferay knowledge. New developers and portal admins without developer resources cannot achieve global navigation.