Auditions are like job interviews. Panels want to assess your skill set against the skill sets of other singers. It's MUCH easier to make those comparisons when the rep is familiar. If you hear an aria you've never heard before, it's very difficult to assess the skill set of the singer because you're so busy assessing the aria.
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"I am always doing that which I can not do, in order that I may learn how to do it." - Pablo Picasso