I didn't fill it out, I feel my Personal statement speaks for itself and says enough about me as a person outside of my stats and such. Don't put anything in that optional part that would just be a repeat of what they can already see on your primary.
I put a short essay just because I already written an optional one for another school and I feel like those are trick questions. Sounds like most people aren't filling it out so I wouldn't worry about it.
I added a short little paragraph about the things I've experienced living in a diverse city like Los Angeles, and how I thought Boston would offer me the same opportunities. Nothing much. Most people I know left it blank. The question about the educational history was more absurd to me, I left that one blank.
i wrote it because i felt like i left out a prevalent aspect of my life that didn't fit in my personal statement or activities page and then i tied it to how that'll make me a better dr. if i didn't have that, i would have left it blank