It is more of an evaluation of leadership potential/abilities than questions about why you want to attend, but there are a lot of teachers (and people) out there that don't necessarily understand how big of a deal the service academies are or treat it like its just another obligatory student rec that colleges don't really care about anyways if you aren't getting anything bad said about you. Its better in my personal opinion if you take the time to explain why this is so important to you and about the Academy. Take the time beforehand so that they understand where you're coming from as well as showing that this is a big deal for you and you are motivated to do this, showing maturity, etc. All of that is important to show to people who are evaluating something that will become a part of your file for admissions