I have served a number of times on grand juries. As I understood it, our task was not to determine guilt or innocence, but rather to see if the evidence seemed serious enough to warrant the person going on trial. There are grand juries which are specifically chosen, however, to do just the same thing Goldstone did; investigate a situation and make a reports of their findings for further action by someone else.
Surfer seriously injured in Australian shark attack (AP)
from the article: Goldstone to attend grandson's bar mitzvah as Zionist groups end feud