Grouping articles into a category is not the same as making a list of articles. When you make a list of articles, you edit the list directly; but when you place articles into a category, you simply edit the articles, and a list of articles is automatically created on the category's page. Despite the difference in how they are maintained, it is sometimes convenient to think of a category as a list of articles.
While an article may be in multiple lists, the goal is that browsing downwards from a list parent category, e.g. Category:Cities, should only arrive at articles that are cities, e.g. London, and not related articles e.g. History of London. See the John Lennon example.
There are some natural hierarchies of lists. One example is the scientific classification of organisms, which would only place an article in one category. Other systems use multiple listings, e.g. Wikipedia could be in Category:Wikis and Category:Encyclopedias.
List categories are likely to also be subcategories of topic categories.
Maintaining and tracking catagories often requires more effort than a simple list. If you have a category that has vague criteria or that adds and removes members frequently, then maintaining a simple list is often more appropriate.
This category currently contains no pages or media.