Can this be one of my acceptable answers for "why do you want to study a PhD"? Obviously, I will mostly speak about my experience performing research at an undergraduate level and in internships and how I wish to undertake a PhD to improve my lab technical skills and contribute to known scientific research.

However, is it appropriate to briefly mention my reason for choosing the University in particular, not merely the research group? For instance, my pet hobby is the piano and I have found a couple Universities, that produce world leading research in my field, that have various performance opportunities for pianists.
When I ask the question "why do you want to study towards a PhD?", the very last thing I want to hear is something entirely unrelated to studying towards a PhD. Not because I fear that you get sidetracked (I don't necessarily worry much about that), and certainly not because I have some inherent bias against piano players, but because I worry that you actually don't know why you want to do a PhD. This is true for a surprisingly large percentage of applicants, and typically a fairly bad sign.

Consequently, if you get this question, I suggest that you explain why you want to do a PhD, and nothing else. If the question is why you want to do a PhD at exactly this university, this can be part of your answer, but it should not be the only, or even the main, answer to that question either. Quite frankly I would imagine this to be largely ignored - playing piano is not memorable enough to make you stick out from the applicant pool, and unrelated enough to virtually all fields of research that I won't count particularly positively either.