The FBI gave immunity to the guy who was the technical admin for her server and email system. Unquestionably, he knew and saw a lot, and he was probably facing very serious charges. He had the keys to the kingdom. Just saying.
He received limited immunity and he was a witness. Clinton is not a witness she is the target. There can never be a reason to grant immunity of any kind to the target of an investigation.
To get immunity, you have to proffer what evidence you have. So the IT guy had something valuable or they would not have given him immunity. You don’t get blanket immunity and then they ask you what you know. You have to tell them first.