When you perform any kind of transformations on the scene or on any object, its transformation matrix (the way with which it is displayed) is modified. When any transformations are performed, they are performed on the current matrix which is then modified depending on the operation to be performed.
Since for that particular code snippet, a clean matrix was required, it was necessary to reset the transformation to identity matrix. Hope it made sense, haven't been in touch with graphics for a long time.
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