The fact that the private server contained classified information is interesting but not central to its purpose. The reason she could not use the secure government server was because everything on it would have to be archived by law and could be subpoenaed by a court. This would potentially expose the numerous quid pro quo discussions she had with donors to her Clinton Foundation while she was Secretary of State.
Because her criminal acts did not create “victims” there are no witnesses that would accuse her of crimes in court. The only way to catch a thief is with a thief and her co-conspirators know what happens to snitches in Arkansas.
“...everything on it would have to be archived by law”
This is it. You nailed it.
I always said to my husband that the real reason she created the server was so that the government would be unable to see it. i.e., have access to it, although I did not know the details on how this process worked. You just confirmed my theory and the reason for it. Thank you!