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<p>Because a lot of times employers, especially universities, like to see that you didn’t just hear the same point of view from the same set of professors for 10 years. Going to a different place for grad school gives you more diversity in your education and that is valuable to a lot of people, particularly at the Ph.D. level. If you are getting a terminal Masters, it isn’t as big a deal because it is only a year or two on top of undergrad so it isn’t going to be as big of a deal if you stay in one place.</p>
<p>There are exceptions to the rule though. If your undergrad school has a professor that is tops in the field and does exactly what you want to do, then by all means, stay on. Some opportunities are just too good to pass up.</p>