To screen out weak cases and protect defendants from facing trial based on unfounded charges, the criminal justice system generally requires the prosecutor to establish that probable cause exists to support their charges. The prosecutor must convince an independent decision-maker—either a judge or grand jury—that the case has merit and should go to trial. In some states, all felony charges must go before a grand jury. But in others, the prosecutor might have a choice between presenting the case to a grand jury or going before a judge in a preliminary hearing.
Key differences exist between these two procedures. This article discusses some reasons why a prosecutor might choose a grand jury over a preliminary hearing (when it's an option). (For more information on grand juries, check out Grand Juries on Nolo.com.)
Unlike a preliminary hearing held in court with the defense side present, the grand jury doesn't make its decision in the context of an adversary proceeding. Rather, it's a one-sided affair.
Grand jurors see and hear only what prosecutors put before them. (While prosecutors technically have an obligation to present "exculpatory" evidence—evidence that suggests that a defendant might not be guilty—there's not much other than the prosecutor's conscience to enforce this rule.) During a preliminary hearing, the defendant can also see and cross-examine prosecution witnesses, which gives them a good preview of the prosecution's case. This situation doesn't present itself in the context of a grand jury.
In part because there's no one on the "other side" to contest the prosecutor's evidence, grand juries almost always return an indictment as requested by the prosecutor. According to a U.S. Department of Justice study, "Grand juries are notorious for being ‘rubberstamps' for the prosecutor for virtually all routine criminal matters." (McDonald, William F., Plea Bargaining: Critical Issues and Common Practices (1985).) It's also suggested that grand juries rubber stamp prosecutors' charges because grand jurors are not adept at evaluating evidence like judges are—making it easier to convince a grand jury than a judge that the defendant should stand for trial.
Prosecutors often prefer grand juries because the proceedings are secret, whereas preliminary hearings are open to the public. The rule on secrecy is meant to provide several benefits. For the accused, it protects their reputation should no charges issue. For witnesses, it's meant to allow them to testify more freely and truthfully. And for the prosecution, it provides control of information. But critics of secrecy rules say it runs counter to the fairness and transparency of the justice system.
Finally, in highly publicized or controversial charging decisions, prosecutors may opt for a grand jury to provide a political buffer. But sometimes this decision backfires. While prosecution offices previously went before grand juries in many police shooting cases, public outcry at the lack of transparency and accountability has resulted in a change of tactics for some prosecution offices.