That is an absolutely silly idea, that identical test scores means identical potential. First of all, after a certain amount, the scores are nearly identical. With the way the tests are graded, one question can be the difference between a 760 and an 800 on the SAT. Can you really say that the person who got the 800 SAT is better? Too many qualities outside of a test need to be considered. What if the 760 grew up in an inner city neighborhood, and was working 2 jobs in highschool to support his single parent? An essay is a perfect opportunity to explain the circumstances of what makes you you and what you have to offer.
Drive, ambition, ideals, character, motivation are all important characteristics in the admissions process at a place at MIT, and they look for people with more than just good test scores. They know that the same test scores can belong to two widely different people-maybe even one they want and one they don't.