I don't think there is one "correct" way to answer this question. Some schools I applied to I really had a couple of distinct reasons why I was applying, so I communicated that in the answer. For the others, I chose to talk about general qualities that the school had (e.g. diversity of students) and how their program is a good fit for my interests. I stayed fairly general on this, for example connecting your interest in research to the school's strong research opportunites, not "i really like genetics and you offer a genetics class". Try to point out how their program is a good fit for YOU by showing them your interests and how they allign with theirs.
There has been some discussion on various threads about whether or not to talk about location as being a factor. Personally, I think when used correctly, it will strenthen your reasons for wanting to go to a particular school. I used it to say "I would like to practice medicine in geographic region X, your school is in geographic region X, this is a plus." (obviously phrased better, but you get the idea) Alot of schools do hope for at least *some* students to stay in their region to practice medicine, so I thought this was a good way to approach why I wanted to be in that state. Just to clarify, this was only something that I wrote when the school did happen to be in the area of the country that I hope to practice in.
