The purpose of a grand jury in the United States is to determine whether there is sufficient evidence to charge an individual with a crime. Typically composed of citizens, the grand jury reviews evidence presented by prosecutors and decides whether to issue an indictment or not. This process functions as a safeguard against unfounded prosecutions, helping to protect individuals from arbitrary legal actions. By requiring a grand jury's approval for serious charges, the system ensures that the decision to proceed with a prosecution is subject to community oversight, adding a layer of accountability in the legal process.