Given the availability of regular personal email accounts (eg gmail, hotmail, etc) and official government email (that she should have been using) the only three reasons for going to the trouble and expense of running your own email server are 1) you're a computer geek and you think it'd be fun and an interesting technical challenge. 2) You plan on selling access to your server and competing with gmail et. al. 3) you want and need that complete control over the archiving and erasure.
Any doubts as to why hitlary did it? Absolutely no doubt there were things in those emails that would have destroyed her public life and/or put her in prison. No other plausible reason for her actions.
Maybe not. The MX records point to MX Logic. That means she's using them as a front-end mail relay, and probably AV and spam filtering. They will have logs of all the emails that passed through their system on their way to, and possibly from Hillary's mail server, and Hillary can't wipe those.