My thought was that Hillary Clinton was worried about scenarios such as Whitewater, which started as a Congressional investigation of a potentially illegal $300K business loan given to a partner of the Clintons by a third party 20 years earlier. Republicans used this to leverage $80 million in taxpayer funds, in combination with supoena power, to conduct a virtually unlimited set of investigations to find political dirt. Thus, long after that $300K loan was long forgotten, and a Benghazi-like train of other false accusations had been raised and debunked, the Independent Proscecutor's team iput a wire on a woman to entrap Monica Lewinski to disclose the affair she had with the President, which then was enough of a political threat that Bill Clinton lied under oath to deny it.
Given this background, it is entirely plausible that Clinton wanted to minimize her exposure to future Republican attacks on false legal grounds. It would have been irrational of her not to prepare for years of such attacks. And while people today are professing shock at her use of a private server, when you look at what was common practice in December 2008, the White House had used a private server, and several of her predecessors had used private servers.
And if you don't remember, the first issue facing senior members of the Obama Administration was that the country was in the worst shape it had been in generations: biggest economic collapse in 80 years (yes, that had international ramifications as well), two ongoing wars, Taliban taking power not only in Afghanistan but also in Pakistani provinces, Iranian nuclear weapons development, etc.
