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No, not necessarily. Consider a graphical representation of a file system, such as the typical folder icon. The individual files are only shown when you open the folder the files actually reside in. Opening a parent folder shows any child folders and any files residing in the parent folder, not the files residing in the child folder.
Imagine if at the root directory of your OS of choice you saw a listing of all the files on the disk at once! It's certainly possible to do this if it's what you need, but it's hardly what you would want to have when you're just looking for one of your mp3 files to play.
Likewise, if I want to read an article about dogs, I'm really not interesting in having to pick through a long list of articles about other unrelated topics. A plain category list is much better for me. That's why you have categories and subcategories to begin with.