When creating subsites, it enhances the user's navigation if links to top-level areas are always shown on the page. The Special:Tools tag allows this. Pages that match the tags in a Special:Tools page will get the content of the tools page embedded in their toolbar area.
This page has its own special tools by dint of its CustomTools tag. There is one other page on this site with CustomTools, and that page also has the Special:Tools tag. So, this page gets those tools.