The Legal Dictionary states that the purpose of a preliminary hearing is to determine in a court of law whether there is substantial evidence to proceed with a criminal trial for someone charged with a felony. A preliminary hearing must be held within a few days after arraignment.
According to Federal Rules of Criminal Procedure Title II Preliminary Proceedings Rule 5.1, a preliminary hearing must be conducted by a magistrate judge if a person has been charged with an offense other than a petty offense, with five exceptions. One exception is if the defendant is indicted, and another exception is if the defendant waives the preliminary hearing. The government can also file information under Rule 7(b) charging an individual with a felony, and a misdemeanor charge resulting from government information is the fourth exception. The fifth exception is the defendant consents to a trial before a judge after being charged with a misdemeanor.