"As we have advised you, we consider Cheryl Mills to be a witness based on the information gathered to date in this investigation. We understand that Cheryl Mills is willing to voluntarily provide the Mills Laptop to the Federal Bureau of Investigation, if the United States agrees not to use any information directly obtained from the Mills Laptop in any prosecution of Cheryl Mills for the mishandling of classified information and/or the removal or destruction of records as described below."
The FBI wanted what was on Mill's computer that involved Hillary, not Mills herself, not personal stuff that would embarrass her if known.
I worked on a case with a lawyer about our county judge being charged by a district judge because porn was on his county computer. The county had no rule about their county computers, for what they could be used. As a result of helping this lawyer, I had to go to the porn websites involved in this case. Those websites captured my information and for years I had porn messages/pictures/etc. When I changed computers, I would have a computer guru switch all info. from the old one to the new one. At some point, I stopped getting the messages/pictures but it could be true that somewhere in the guts of my main computer, those are still there. I can explain how they got there, but if publicized, would anyone believe me? Oh, yes, those websites were all men, no women. They were homosexual web sites.
What is on your computer you do not want the public to know? That is why Mills wanted immunity and the info. destroyed.