I've recently received several questions about optional secondary essays. How to approach these depends on two things – the prompt itself and your candidacy.
The beauty of a generic "additional comments" section is that it is intentionally vague. It's your chance to provide details, context, or qualifications that the structure of the application didn't allow you to present.For that reason, if you have a candidacy without any red flags, I lean toward using the "additional comments" space to both highlight who you are and any exceptional aspects of your candidacy that you'd want a reader to know before making an interview decision. One good option in this circumstance is to pick something completely nonmedical that distinguishes you and is nowhere else to be found in your application. Since an interview isn't guaranteed, don't save your best material for an in-person meeting. (On the other hand, if you have a large weakness in your candidacy, you usually want to use an optional, generic prompt or a more focused one to gingerly address the issue. See the next paragraph for guidance.)
Sometimes an optional essay is more pointed. An applicant recently forwarded me this prompt: Please describe any extenuating circumstances that may have affected your medical or non-medical service experiences, including any circumstances that impacted your engagement in activities, academics, and MCAT that would have helped to prepare you for medical school. If you have no major deficiencies in your candidacy, there's no need to write this essay. On the other hand, you should draft a response to this prompt if there's a big elephant in the room. While, in general, I tell applicants to avoid highlighting standard weaknesses, sometimes someone has a big problem that's important to address head on. It's better that you write your own story than let someone else do it.
Bottom line: Optional essays are frequently worth completing because this process is so competitive. And, if you have a big weakness, you should leverage an optional prompt to explain extenuating circumstances.
