As I understand it - IANAL - the grand jury decides if there are crimes that have been committed, and if so, who should be charged with what.
Let’s pick on the Breanna Taylor case - not that I want to stir anything up, but it’s a really good example of the process. Police exercising a warrant break into a house, gunfight ensues, someone dead. What charges should be laid against whom?
The DA could simply lay charges, but charges apparently carry more weight if an independent “jury of peers” hears the evidence surrounding the event and decides on charges instead. Plus, the DA has to live with the police and rely on them for all the cases he prosecutes, so letting the grand jury decide also removes the blame that he would be “picking on” police if charges are laid - or allows him to blame the grand jury if charges are not laid. “They looked at all the evidence and decided no charges were warranted…”
The grand jury proceeding is secret, so what is said there (usually) is private under severe penalties, so testimony given there does not taint the possibility of a fair trial. It’s also said the grand jury is a fishing expedition. The DA can drag people in front of it, and see what they say about an event. Once they testify, witnesses who change their story later are committing perjury. If they plead the fifth, the prosecutor has the option of giving them immunity. With immunity, a witness is no longer at risk of self-incrimination and must testify.
The joke is that a grand jury will indict a ham sandwich if the DA asks them to, since the jury only hears the DA’s side of the case. Witnesses are interrogated for information by the DA, but the persons possibly being indicted are not present, don’t hear anyone else’s evidence, and certainly don’t get to cross examine to pick holes in what the DA is revealing.
What is interesting about the Taylor case is that the prosecutor did try the excuse “the grand jury said no charges should be brought”. Grand jurors objected, claiming it hurt their reputation and they needed to disagree - and the judge allowed the record to be made public, showing the prosecutor specifically told them certain charges against police should not be brought contrary to what he told the public.
But to answer the OP’s point - to arrest someone, you need probably cause. then to hold them, you need to charge them, IIRC within 48 hours, depending… (At which point they may or may not be able to get out on bail) As mentioned, the prosecutor can simply lay charges if he feels there’s enough evidence. If there’s a grand jury after this, it’s probably to examine further actions of the perp, or examine the actions in depth, to determine if more charges should be laid or those charges should be changed. After all, the purpose of a grand jury is to determine whether charges should be laid. If charges are already laid, the grand jury is redundant with respect to those charges.
Usually, grand juries are to examine complex cases to see who should be charges and who shouldn’t. The subject(s) of these investigations already know things are rolling; if they intended to flee, if they could have and should have, they would have. Plus, revealing that one has been summoned to testify in front of a grand jury is also forbidden until the indictments are public.
For example - news stories do not mention whether Bannon was indicted by a grand jury, or the DA simply laid charges; but the fund’s bank records were subpoena’d a year ago, so anyone with a clue would have known the feds were looking into it.
I assume - maybe a real lawyer can confirm - for fairly simple, obvious cut-and-dried cases a grand jury indictment is not needed.