In rare cases USCIS schedules an interview.  This is to determin if the marriage is real and ensure that there is no immigration fraud in obtaining the green card through marriage.

USCIS asks questions to check if:

the parties to the marriage are in a genuine relationship ;

the foreign national is being truthful or lied about an important fact to obtain greencard.

the foreign national paid any money to receieve the greencard;

Marriage Fraud

The I-751 interview gives USCIS an opportunity to inquire deeper into the circumstances of the parties' life and investigate more in suspect cases.

A decision may be withheld for more investigation when necessary.

It is therfore very important to present a well prepared packet and answer all interview questions asked by the officer.

Standard of proof and appeal

USCIS frequently mixes up absence of marriage bonafides with proof of fraud. 

However, this distinction is subtle.

Finding of marriage fraud is difficult to overcme.  It is therefore necessary to have a well documented case.

Appeal against marriage fraud denial can be filed before the  Board of Immigration Appels (BIA).