Grand juries are actually meant to protect the defendant.

It's a requirement that the prosecution present its case pretty much without challenge to see if they have the evidence to prove everything they allege. The idea being that if you can't convince a jury without the defense poking holes in your case, it's not strong enough to go to trial.

That's the theory. What I have seen in practice is that because the defense has a very limited ability to object, the prosecution will present misleading or easily refuted evidence to the jury in order to secure the indictment. Such evidence can be easily rebutted at trial, but the process is the punishment, and they are effectively circumventing a check and balance to keep the prosecution active.

Other legal systems have a similar screening tool. I believe in Spain they have to present their case to a judge who decides whether or not it has enough merit to move to trial.