Can anyone explain the reason for these? I see lots of downsides and no upside.

Grand jury decides if there is enough evidence against a person to merit going to trial. It saves a lot of money.

A friend of mine served on our county grand jury for 6 months (or maybe it was 3?) and told me about it. I think they met about every 3 weeks. Its basically the one side of a potential trial and usually the state DA gets their way because only they present their case. However, my friend did tell me that once and while the GJ would tell the DA., "no, you can't go forward with it."

Its not much of a check and balance against a powerful administrative state but it is some. Comes from the old English legal system that was brought to the colonies.

The Grand Jury is not an all or nothing decision, just takes a majority to send to trial. Having served once for six months, I can confirm, the saying “a grand jury would indict a ham sandwich”, is true. lol

Most if not all that transpires in a GJ deliberation is secret, so the defendant likely doesn't even KNOW they're under investigation.

There may be a valid reason for it, but seems to me it runs the risk of becoming useful tools by over the top prosecutors. Treated like mushrooms.........kept in the dark and fed a steady diet of horse droppings. But never sat on one, so don't know process.

Still not sure what benefit they have, or can do that a prosecutor can't do on his own?

This ^^^

I don't handle criminal defense but sometimes my civil work intersects with grand juries. We generally find out about the investigation one way or another. But I work with companies, not mobsters--and my experience with grand juries is limited.