The optional essay is always good to remove concerns from potential weak areas in your profile, or if you'd like to highlight reasons for periods of non-employment, etc. you could really do anything with this essay including highlighting other factors on why columbia is the right fit, or why applying late in the cycle, etc. , etc.
i just submitted my app on monday, and in my optional essay i highlighted my reasons for applying now, and how i would've applied ED if i had been under different circumstances. i basically weaved another story into this last essay that also showed why columbia is my first choice for both me and my family, and that i'm visiting the school for the host program in march. lastly, i made a note about some areas on one of my transcripts.
maybe i went overboard with it, but they will have no doubt columbia is my first choice.