The Justice Department's Hillary investigation reeks of corruption

Question is: Does the Justice Department regularly grant immunity for mere cooperation? After all, Mills, Clinton’s former chief of staff, was offered protection in exchange for something any non-guilty person would freely offer. According to a number of reports, she handed over her computer. These people persistently assure the public they’ve done absolutely nothing wrong. Why would Mills need immunity?

Why would the government give it? What if they’d found something? If Mills was the beneficiary of immunity before the FBI even knew what was on her computer, what pressure would prosecutors be able to exert to impel her to cooperate against the (pretend) target of the investigation? And if they didn’t think they’d find anything, anyway, why grant immunity in the first place? What made Mills so special?

And how often is a political operative like Mills granted immunity and then comfortable enough to represent the target of the investigation in an interview with the investigators and in front of Congress?