Two tiny words that carry so much weight, but to be fair, if I were a college, I’d want to know too. The thing about the Common Application is that it’s universal. You are writing an essay to send to every school. It answers the question of why you, but doesn’t touch on anything specific to the school that receives it.
As a college applicant, you will likely be asked to respond to an essay question that asks “How will you take advantage of the academic opportunities at our university?” These questions fall under the category of intellectual interest essays and the intent is not just to focus on the major you have selected or your favorite high school course.