Except she could have done that with her .gov email. Classification laws state she can delete personal emails from her .gov account without fear of wrongdoing. So your theory falls flat.
One reason she might have wanted to setup her own private email server was to skirt FOIA requests. Which seems plausible since there were numerous FOIA requests during the Benghazi ordeal. All of which were replied to with 'we have no emails'. It took Syndey Blumenthal's AOL account to be compromised for everyone to discover Clinton's private email server.