A main function of citations is to lead readers to an original source. In academic writing, bibliographies and footnotes (a note at the bottom of the page that cites a source) make it relatively easy to track down information sources.
In journalistic (also called "popular") writing, information sources are often identified incidentally. For example, throughout a magazine article, the author might refer to the author, journal title, and publishing date of an information source, but not include a bibliography. This makes tracking down the source more challenging, though still possible. Sometimes the original source is hyperlinked, in online sources. Here’s an example from the New York Times and the paper it is based on: