How Important Is Optional Or Supplementary Application Material?

Optional or supplementary application material is pretty self-explanatory in the sense that it is, well, optional or supplemental. It is there to give you the option to supplement the information already present in your application with other details about yourself that you think might be pertinent to an admission office's decision. If you haven't found a place in your application to share an interest, pursuit or accomplishment, this is your chance to do so. It never hurts to provide admission officers with additional information on your strengths and skills.
What Should I Use As Supplemental Materials?
You've most likely covered the basics with your written materials in your applications, so the optional or supplemental materials can be a great way to share something visual with admission officers. This can be in the form of an art portfolio, a video of a theatrical performance or a debate team competition. Keep in mind that using some form of visual media has the additional value of helping you stand out in both the applicant pool and in an admission officer's memory.
Don't Repeat Yourself
It's also important to remember that the supplemental material should not be used in a way that might be redundant with other parts of your application. If you're required to submit an academic writing sample with your application, you don't need to submit a second sample in your optional materials. If you've won academic awards or athletic competitions and these are listed on your application or appear in your essay, there's little benefit in restating these achievements here.
Your best bet when deciding what to include in your application will be to keep your application to the requirements and only including additional information when it helps offer a better image of yourself. Admission officers are incredibly busy during application season, and most aren't able to give optional/supplemental material more time and attention than the average application receives.
Think of it this way: If your application feels like an accurate picture of you, then you can feel satisfied that you've included everything you need to submit. If this is the case, it's complete and you can rest easy knowing you've done a great job and don't need to add anything extra. If you do want to submit additional material, make sure that it follows these three guidelines: That it is adding something new to your application, that it can be understood quickly by busy admission officers and that it follows the school's directions for submitting material to a T.