Here's the prompt:
"Please provide any additional information or background you believe will be of assistance in evaluating your application. (400 to 500 words, 3000 characters max) "
You can write about anything you want in this portion. Talk about important extracurricular activities, athletics, or something else. If you are writing about wanting to attend the academy I would suggest talking about how the academy aligns with your personal goals or how attending such an institution would help you reach your fullest potential. Those are just some examples but you want to make your essay's unique and interesting to read instead of writing something like "Going to the Academy would mean a lot for me because I have always wanted to serve my country." That response has probably been read by the admissions board thousands of times and it is a good reason, but not very unique.