You know why you typically grant immunity to a witness, yes? You do it because you need the information they can give you in order to reel in the really big fish, but they're reluctant to give it because the information will likely incriminate them too.
So when the FBI is handing out immunity left and right, what you can tell is that lots of people are terrified of what will happen to them if they tell the truth. But the FBI wants that truth so badly, in order to nab the real subject of the investigation, they agree to the deals regardless.
In the case of the Hillary e-mail investigation, the question we're left with now is whether anyone wasn't granted immunity. We learn today that even her top crony Cheryl Mills, who sometimes calls herself Hillary's lawyer when it gets her out of answering questions, was given immunity. In exchange for what? Letting the FBI read her e-mail, it would appear: