Many colleges offer optional essay topics in addition to their required essays. How do you figure out which optional essays are truly optional, and which ones are actually must-do's?
One quick way to distinguish is to look at the nature of the optional essay question. Is it asking some version of "Why are you applying here" or "Why are you a good fit for our school?" If so, it's effectively asking, "Why Us?" Northwestern's optional essay topic is a good example. Here's the prompt:
Other parts of your application give us a sense for how you might contribute to Northwestern. But we also want to consider how Northwestern will contribute to your interests and goals. In 300 words or less, help us understand what aspects of Northwestern appeal most to you, and how you'll make use of specific resources and opportunities here.
What they're actually asking is "How badly do you want to go here?" If you don't submit a thoughtful essay in response, you're essentially answering: "Not very much." And that's a quick way to get rejected.
What those schools are showing you is that they are very sensitive to who is genuinely interested in them, and they really want to know what specific things are drawing you to that college, for example if you're really interested in their dance program and want to participate in their dance club. Be specific!
(The admissions folks at Northwestern are especially nice because they even tell you at the end of the essay prompt: "We HIGHLY recommend you complete this essay." Not all colleges are that blunt, though.)
Other kids of essay topics are truly optional, and it won't benefit you to submit something just because you can. For example, Harvard has a looooong list of optional essay topics, but not a single one of them is some variety of "Why Us?" (It's nice to be Harvard; they assume you want to go there.) That's how they're signaling to you that the optional essays are truly optional. If you go over to the Harvard admissions website, they even tell you as much in their Application Tips if you click on the little "i" icon:
There is no “extra credit” for writing this optional piece. As you are filling out the application, if there is not a topic that naturally comes to mind then you should probably skip this question.
So a good rule is that if an optional essay topic is asking some version of "Why Us?", definitely write something (and make it good).
And if the optional essay question is not a "Why Us" essay prompt, submit something only if it's a strong piece of writing AND it says something about you that isn't already demonstrated somewhere else in the application. For example, if the optional essay question is "Is there anything else you want us to know about you," don't use that as an opportunity to say to them, "Oh by the way can I mention that I received an A+." That won't add anything valuable or new to your application, and you're better off not submitting the optional essay at all.