They are a backup for the naturalization certificate. Usually, they use the digital photo that is taken for biometrics after filing the Form N400. If the equipment for making the certificates is not working, they sometimes use the backup photos.
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A variety of reasons including verification that he same person is submitting the application is the same person who was granted residence, completion of naturalization certificates, etc.
While this answer is provided by a Florida Bar Certified Expert in Immigration and Nationality Law, it is for general information purposes only and an attorney/client relationship is neither intended nor created. You should seek out qualified counsel to review your case and provide you with advice specific to your situation. Review Mr. Devore's Avvo Profile for more information about his expertise in immigration law and how to contact him to discuss your case.