@gollum,
On my second term it was understood that it was to protect us-and of course keeping our names out of the public until it was all over.
When we investigated the prison system we just showed up and no one had the slightest idea who we were.
The grand jury process can be tweaked in a number of different ways-and I'm sure it different from district to district.
As far as your question we had sealed indictments in cases that still needed further investigating and it was to protect the integrity of the case and not identify street informants and other hearsay evidence that is not allowed in a criminal trial.
To understand how this works you really need to be there and see it for yourself. By the end of the 5th month we just wanted to get the hell out of that room. What people don't understand is that 98% of the cases are just boring and routine like DUI and it begins to get on peoples nerves day in and out.
Sorry, I can't answer any better but that was more than 20 years ago.