Unlike a preliminary hearing, held in court with the defense side present, the grand jury does not make its decision in the context of an adversary proceeding. Rather, grand jurors see and hear only what prosecutors put before them. (Prosecutors technically have an obligation to present "exculpatory" evidence—evidence that suggests that a defendant might not be guilty—though there is not much other than the prosecutor's conscience to enforce this rule.)
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 on plea bargaining, "Grand juries are notorious for being ‘rubberstamps' for the prosecutor for virtually all routine criminal matters." (Plea Bargaining: Critical Issues and Common Practices, by William F. McDonald, (U.S. DOJ, National Institute of Justice, 1985).)
Where they have a choice, prosecutors often prefer grand juries because grand jury proceedings are secret. When prosecutors file an information, they are usually required to convince a judge in a public preliminary hearing that they have enough evidence to secure a conviction. This high burden is not present at a grand jury proceeding, where the prosecutor has to prove that there's only "probable cause" to believe that a crime occurred and the target of the proceeding comitted it. Also, during a preliminary hearing, the defendant can see and cross-examine prosecution witnesses, which gives them a good preview into the prosecution's case.
This article was excerpted from The Criminal Law Handbook, by Paul Bergman, J.D., and Sara J. Berman, J.D.