<p>
</p>
<p>Ah, well quite simply because in some departments, the culture just is such that they want you to have exposure to different locations, as they generally have different kinds of faculty to get you exposed to, and may quite possibly have different research cultures. I feel like part of the reason the math department cares more about this is that it’s SO small at most top schools, and it’s plain that you’re actually going to feel the difference noticeably going from one school’s department to another’s.</p>