Why do you want to go to Cambridge?

Put yourself in the interviewers shoes. They have a limited number of spaces, so they are trying to pick a candidate that can firstly put up with the workload of Cambridge, but after they have cleared the benchmark, they next want to pick a candidate that is most likely to benefit from Cambridge and contribute to the Culture at Cambridge.
So in answering this question, firstly think about who your interviewers is; Are they a tutor (non-subject) or a DoS or lecture (subject related)
For subject related, focus on points about why you love the subject, what is special about Cambridge in teaching you the subject, what about Cambridge do you hope will develop you the most, and then maybe what you can hope to give back to Cambridge, in terms of the subject.
For non-subject related, focus more on the university culture, what attracts you to Cambridge regarding its culture? How do you hope to contribute to the culture? What do you hope to gain from the non-academic aspects of Cambridge?