Grand juries are a holdover from the early British common law dating back to the 12th century. Deeply rooted in the Anglo-American tradition, the grand jury was originally intended to protect the accused from overly-zealous prosecutions by the English monarchy. In the early phases of the development of the U.S. Constitution, the Founding Fathers decided to retain the grand jury system as a protection against over-zealous prosecution by the central government. Although the Supreme Court in Hurtado v. California in 1884 refused to incorporate the grand jury system into all of the states, most states have independently decided to retain a similar form of grand jury, and currently, all but two states (Connecticut and Pennsylvania) have the grand jury.
https://www.law.cornell.edu/wex/fifth_amendment