Basically I would take it as you were deferred, but you are ranked a bit higher among the deferred applicants. In this case, they want to see who is serious about attending and who is not. So maybe somebody who got into another school of choice might just be like forget it. You should write one if you are genuinely interested in attending, not if an acceptance would be just for boosting your ego.
If you do choose to write one, think of it as two parts. What attracts you to the university, and why you think you are a great candidate. You should not just regurgitate your application, but try to think about maybe what updates you have since submitting your application.