Why did the District Attorney's office convene a Grand Jury?

[gumshoe1]

It's my understanding that many years ago, the Boulder County District Attorney's Office convened a Grand Jury, and the Grand Jury indicted John and Patsy Ramsey on charges that were something like child endangerment or something instead of murder. But the Boulder County District Attorney's Office declined to prosecute John and Patsy on the charges that the Grand Jury indicted John and Patsy on. If the District Attorney's Office was not going to prosecute John and Patsy on the charges that John and Patsy were indicted for, why did the District Attorney's Office convene a Grand Jury in the first place?