I agree with Dyingduck. I saw this question as more of - "what makes you different from the other applicants." I answered this question by summarizing my favorite EC's and how they were going to help my future as a med student and as an eventual physician.
Sometimes it can be hard to answer questions like this one... as I think we're typically raised to be humble. But this is your time to shine! When I interviewed at AZCOM, I was specifically asked "how confident of a person are you on a scale of 1 to 10?" and the "why should we accept you" question. Again, as Dying duck mentioned, this essay question is good practice for the interviews... .
Best of Luck