Seems to me the reasons for her decision to use a private server for government business are pretty simple. It means that she (and her staff) get to decide which documents should be forked over in response to FOIA requests.
In a just world this server would now at an independent expert for thorough inspection.
Same thing for congressional oversight. Case in point: Benghazi.
Also, it keeps all of her correspondence out of the official protocols. She wants to delete some stuff? No problem. That would be more complicated if she had used her government-issued means of communication.
I seem to remember from earlier incidents (like the hack of Sarah Palin's personal mail) that this is *not legal*. For good reasons.
Finally, it is basically a given that some of her correspondence contains sensitive, if not outright secret, information. If someone like Thomas Drake gets threatened with ridiculous punishment for having *un*classified information on his home PC, surely this here should land Mrs Clinton in a whole lot of trouble. But, well, who am I kidding, right?