Headers, footers, notes, comments, alt text, probably dozen of other features. Documents often are lot more than just markdown so properly to support everything you do have a lot of ways to hide text for various use cases.

These are types of text that are, to some extent, effectively hidden. But I don't think that's what the article is talking about.

Word has a feature literally called "hidden text". Select some text, go to the font properties dialog, click "hidden" and OK, and watch the text disappear.

Edit: actually, this is white text on a white background as others have said, not true hidden text.