To allow related articles to be grouped together, and those groupings placed into a hierarchy. For example, both [Gwalior] and History of Gwalior could be in the Category : Gwalior.

There are some natural hierarchies of topics, e.g., placing Category:History of India under Category:Asia and Category:History.

Topic categories are unlikely to be subcategories of list categories.

Currently the software does not allow you to look at Category:People and retrieve a list of all articles in sub-categories.