This template is for use on Category pages, to refer to the primary article associated with this category, which most of the time, describes the entire set containing the category.
This template is not a replacement for adding a category to the main article. The main article of a category should always have the category set. Furthermore, the main article should always have a space as sort key so that it appears on top of a category. Example: the article Internet should have the category [[Category:Internet| ]].