There are a number of advantages and disadvantages of the grand jury system, which represents a major component of the criminal justice system in the United States.
Criminal trials, especially those involving major crimes and conspiracies to commit crimes, are financially costly to the governments, state or federal, that must foot the bill. The main advantage of a grand jury, consequently, is that it provides a system for conducting a legally-binding "dry run" before a formal and protracted criminal trial takes place. Prosecutors will take a case before a grand jury as a way of determining whether sufficient evidence exists to advance to a trial. In a regular criminal trial, the jury determines guilt or innocence based upon the preponderance of evidence before it. Grand juries do not determine guilt or innocence; they determine, once again based upon the evidence presented, whether a case should go to trial.
From the prosecutor's perspective, a grand jury can be an invaluable tool in convincing potential defendants to plead guilty to the charges, to enter into an agreement with the prosecutor's office to cooperate in exchange for a reduced sentence, or to bring out information that might not be admissible in a trial. In other words, the grand jury system provides the government a way of manipulating the criminal justice process through the mere threat of bringing individuals before the grand jury. Because it is illegal to commit perjury before a grand jury, and because the prosecutor has more discretion in how to question witnesses than in a regular trial, the risk to the individuals subpoened to appear before the grand jury can seem very high.
Another advantage of the grand jury system, again, from the perspective of the prosecution, is the perception of impartiality on the part of a grand jury when they actually serve as an instrument of the prosecutor's office. While a judge selects and swears in members of both regular and grand juries, the latter are not beholden to the principles that undergird the trial system, mainly the presumption of innocence until proven guilty. Because grand juries do not determine guilt or innocence, but rather only whether sufficient evidence exists to issue an indictment and proceed to a trial, there is less of a burden on them to demonstrate impartiality.
The disadvantages of the grand jury system, therefore, tend to reside with the defendant and his or her legal representation. Because the mere process of issuing subpoenes to individuals to testify before a grand jury is frightening to most suspects and possible conspirators -- especially for suspects granted immunity from prosecution in exchange for testifying against other suspects, thereby depriving such individuals of the right against self-incrimination -- defense attornies tend to dislike the grand jury system.
As noted, the threat by prosecutors of taking a case before a grand jury is a great motivator for the defense to seriously consider its options, including cooperating with the government in exchange for a reduce sentence or in exchange for the prosecutor's office dropping its investigation against the newly cooperating witness.