Why isn’t the URL enough for citation? The short answer is, because URLs can change. Websites undergo redesign all the time, and URLs often change without notice. Think of it as moving without leaving a forwarding address.
We cite our sources for two main reasons: one, to give credit to the source of your information (to avoid plagiarism); two, to make it possible for others to go back and find that source. If the only piece of information provided is a URL, and that URL is changed after you use it, this prevents others from verifying your source, resulting in plagiarism, which is a serious academic offense. In order to make the source “findable” you need to provide key pieces of information that don’t change. If you have also provided the author, title, date of publication, etc., it gives someone enough information to search for and find the source you used, even if it moves.
While you should always provide the URL for an online resource, you must include other pieces of information so that if that URL should change, that source could still be found. The format for a citation changes based on your field of study – you’ll learn MLA format in ENG 101 and 102, but other courses might ask you to use APA (social sciences), Chicago (history), AMA (medical), or others.