Whether or not you should answer an "optional" secondary essay prompt depends on the question being asked. If it’s targeted, like asking you to explain a gap in your education, and you don’t have one, you can skip it.
But when schools give you an open-ended prompt such as, “If there’s anything else you’d like us to know,” it’s usually worth responding. These responses can help admissions committees better understand you, highlight strengths not covered elsewhere, or explain weaknesses that need context. Your decision to answer or skip it might influence whether you’re invited for an interview.
Use this space wisely. Talk about meaningful personal experiences, unique perspectives, or anything significant that hasn’t been covered. Optional does not mean unimportant.
That said, don’t write an essay to fill space. If you have nothing relevant to add, it is better to leave it blank than to submit something that feels generic or weak.