"A grand jury is a legal body that is empowered to conduct official proceedings to investigate potential criminal conduct and to determine whether criminal charges should be brought. A grand jury may compel the production of documents and may compel the sworn testimony of witnesses to appear before it. A grand jury is separate from the courts, which do not preside over its functioning." Wiki
Grand Juries were first created in English law in the time of Henry II. He was the first Plantagenet king of England (and of about half of France if you include his wife's posessions in Aquitaine). The purpose of the GJ has always been to limit the sovereign's ability to try people arbitrarily in whatever form of proceeding he liked without regard to the likelihood of an actual crime having been committed. The idea is that sovereigns can be influenced by; reasons of state, the demands of mobs, sheer capriciousness or any number of other factors. The Grand Jury exists for the purpose of giving the "peers" of the suspect a chance to tell the sovereign (or his representative - the prosecutor) that there is not enough evidence to think the suspect BROKE THE LAW.
The issue before the GJ is not whether or not the suspect made bad decisions, did stupid things or is an unpleasant object. The issue is simply a sufficiency of evidence to make it likely that the suspect BROKE THE LAW and therefore a trial by jury is justified.
It is evident that many people do not understand or accept the function of the GJ. There are a lot of ignorant people in the US. Many of them have no conception of the structure or functions of government. They tend to operate on emotion rather than reason and in situations like Ferguson and NY City the process is of no interest to them unless the result is something they like. If they do not like the result there are demands for the abolition of the present rule of law in favor of what? Lynching? Arbitrary prosecutions by the states or federal government?
Lawyers like to say that a GJ will indict a ham sandwich if led in that direction by the prosecutor. That is simply not the case however much lawyers would like to believe in their Svengali-like powers of persuasion. I served on a Virginia GJ 10 or 15 years ago. It met on and off for 6 months and heard many, many cases presented. We refused to return an indictment in 2 of those. The 1st time it happened the Commonwealth's Attorney (state prosecutor) lectured us about how hard his people worked to assemble evidence, how sure they were of the justification for a trial, etc. He then waited for an apology. One of the jurors asked him if it was his belief that we should vote however he wanted us to vote. He left the room quickly in the sure knowledge that if he answered the question wrongly the GJ would speak to the Attorney General of Virginia.
At the first and last meeting of that Grand Jury a state district court judge appeared briefly to remind us that if we knew of or suspected any other criminal behavior we had the right to open an investigation into the matter.
I like Grand Juries. pl