The way I see it, the point of the essay is to provide more information about yourself beyond what you have already filled out in the online application and on your resume. For example, on my resume I list the improvements I made while at my various jobs, I also gave a very very high level view of what the job entailed. In my essays, I built upon one or two of the bullet points I had. So if on my resume I listed something like "Increased productivity of 20 analysts...blah blah blah" I would then in an essay state what actions I learned about myself through that process, and how that made not only the organization better, but also me.
...that is the approach you want to go with...stay away from repeating thigns, like your birthdate, that are already in the app...we only have so much space to write with.