Yes a grand jury is just the prosecutor and jury. It's secret and the defendant doesn't even know. It's a way to start a case, not sure when a prosecutor needs or doesn't need a grand jury.
• High public interest in the case;
• The fact that a preliminary hearing would take more time than a grand jury hearing;
• The necessity for calling children or timid witnesses who would be subject to cross‑examination at a preliminary hearing;
• The ability to test a witness before a jury;
• Where the secrecy of the grand jury may allow defendants to be charged and taken into custody before they can pose potential danger to a witness' safety or flee from the jurisdiction;
• Where the identity of undercover agents needs to be protected;
• The existence of a weak or doubtful case which the district attorney wishes to test;
• The opportunity to involve the community in case screening; and
• Whether the case involves malfeasance in office.
https://www.pooleshaffery.com/news/2014/december/a-crash-cou...
I wonder how other common law systems handle this (not a lawyer, am genuinely asking).