Template:Formatnum/doc

This template doesn't do an awful lot on its own. It's basically just an error message. It displays when people accidentally invoke a magic word that has the same name as a template. It explains that magic words are called when a straight line (|) is used after the name, whereas a template requires a colon (:)

In other words, as is explained in greater detail at the MediaWiki site, there is a big difference between these two statements:

{{DEFAULTSORT:Last name, First name}} <-- template
{{DEFAULTSORT|Last name, First name}} <-- magic word

This template can thus be used to inform a user that they've used the wrong format for their purpose.