The purpose of the grand jury system is to ensure that innocent people don't have their names dragged through the mud by the mere fact of being accused of a crime. The US founding fathers were concerned that the government could announce that, say, they were indicting ein for murder, let that accusation ruin your reputation by being reported in papers, and later withdraw the charges when it turned out there was no evidence. The accusation would still sully your reputation.
The grand jury system avoids that by forcing the prosecutor to lay out their evidence to the grand jury in secret. These proceedings are sealed so that your reputation isn't damaged if the grand jury declines to indict. If the grand jury does choose to indict, that indictment stays sealed until the prosecutor makes it public.
Prosecutors may keep indictments sealed for many reasons. But in the case of large, long-running investigations, the most common reason is so that they can unseal indictments against a number of people at once. If you're investigating an organized crime family, for example, you might develop evidence against Little Fish first, go to the grand jury, and indict him. With that sealed indictment, you might convince Little Fish to turn state's evidence and implicate Medium Fish. You go to the grand jury and secure an indictment against Medium Fish and leverage that, eventually, to indict the Big Fish you're after. At the end, you unseal all the indictments at once and arrest everyone. If the indictments were made public immediately, it would be a major tip-off to the organized crime family that investigators were closing in on Big Fish.
