The why us essay is really about showing colleges what YOU are going to contribute to their community, and what opportunities you are excited about. They vary in length from 100 words to 650, so how in-depth you can go will vary.
It is important to tell them why you think the specific college is a great fit for your major and your career goals. Try and mention programs and opportunities that are unique to the school and how that fits into your future goals. It could be research opportunities, unique core curriculum, certificate programs, etc.
Then you can talk about extracurriculars you are excited about - again always tying back to how YOU will contribute to campus. This could be sports, clubs, volunteer opportunities, cultural organizations. The more specific to the campus the better. It is an opportunity for you to tell them about your personality and how it will fit into their campus culture.
I have heard again and again from AO that they don’t want to hear about the city or the weather or general points that are not unique to their school.