USCIS requests applicant to bring his ID to the citizenship interview!!

Asked in Chicago, IL - about 1 year

A friend is due for a naturalization interview soon and he was concerned when he received from USCIS a "Pre-Interview Case File Review" letter printed on a yellow paper informing him that they have reviewed his file and his N-400 application and needed him to bring his state issued ID to the interview. It is given that every one should carry his ID with him however does any one knows why USCIS makes this explicit request to bring the state-issued ID to the naturalization interview? and have any one received this letter, attended his citizenship interview and found out what is the reason behind this strange request?