When you’re working with site templates in SharePoint Foundation, you might want to group the links to lists and libraries under different headings, than their type. By default, SharePoint will create a heading link “Libraries” and list the libraries under it. But when you’ve got a lot of libraries, you might want to group them under different labels.
But any header link in the quick launch requires that you specify a URL from the web application’s domain root, i.e. “/sitename”. You do not always want this, if you’re simply grouping links in the quick launch, having a link there is not always wanted, especially if you want to create a site template from the site.
This link will still be valid for SharePoint (it requires that you enter a URL), but won’t cause any navigation to happen for the user.