Not only are you writing an essay that explains why you want to attend a particular college, but you’re also writing an essay that explains why you want to major in a specific subject.
You should mention both, and you should tie them together. For example, suppose you are applying to a school that offers marine biology degrees. In that case, you should write an essay about how excited you are to be studying marine biology, how you’ve always been fascinated by the ocean and marine life, and how this degree will give you the perfect opportunity to pursue that interest. When colleges read your essay, they want to know that you really want and need this education. They want to know that you’re not applying for admission to any and every school just because you have to and that you’ve thought carefully about which schools and degrees are right for you.