Her inbox containing inbound email was probably the most damaging. The inbox contents might indicate direction was given to her to engage in political activity.
The outbound email could demonstrate the directions she was giving to others. It could also include reports to her superiors apprising them of her activitites.
What record of incoming messages that were long since scrubbed would there be at this point? When overstuffed email accounts were getting crowded, agencies would automatically delete old emails after X amount of time. If there any proof that the computer(s)/devices she uses today are the same ones she used as Sec. of State? Highly doubtful. Would the international networks over which her messages travelled be able to track down messages, if they knew the IP address(s)? Unless recipients and senders of emails to Huma Abedin (probably highest Muslim Brotherhood operative in State Dept under Hillary), Bill, Hillary, and the other top aides to Hillary come forth and declare that they had conversations online, what other mechanism would there be to come up with that info? . I can only assume that her address books weres long since deleted/laundered.