What was the point behind her setting up a private email server anyway? Like why was that necessary?
Emails of government officials are public record. They can be accessed using FOIA.
Clinton used a private email server to conduct state business. This mixed personal emails with cabinet emails that she created and received while Secretary of the State.
Conducting business using a private email server means you can delete and control what emails any oversight committee or agency can see. It's like using a burner phone rather than the business phone.
If Clinton was doing anything shady like I assume most politicians are constantly doing, this would give her more control. It limits the damage in the event that something like this happened and prevents things from leaking that would be politically difficult to defend.
The other issue is that if the emails she was creating and receiving were related to classified issues, conducting said business outside of the government using a third party, would be illegal.