Federal law requires that federal employees use federal e-mail services while they are working and while they are representing the US Government. Hillary did not. She used a commercial service.
Federal law requires this to allow the government to control the content of all e-mails these employees send because the government wants to be able to monitor their communications and take the appropriate action in the event of some violation of personnel policy.
The government wants to be able to archive official e-mail communications so they can be produced in response to things like FIOA requests or Congressional subpoenas. Hillary's use of a private e-mail even for official communications and communications with other employees violated this law and frustrated these purposes. More here.
Clinton destroyed a great many e-mails that should have been available for Congressional investigation and would have been if she had followed the law. And she's saying, "At this point, what difference does it make?"
From a security perspective, there was none. Her internet service provider had access to every sensitive e-mail that she sent and received. What foreign country owned her Internet Service Company? What foreigners could read and resend her e-mail? I don't know the answer to those questions, but was Secretary of State Clinton conniving (clearly) or simply guilty of crass stupidity? She's a lawyer - she tells us that she is a great lawyer - and she didn't know that there was a law against using commercial e-mail for official correspondence? Really? What about that video?
We know that she was dead broke because she's told us that she was. How much money did she bring in through private e-mail solicitation while Secretary of State? We'll never know because she deleted her e-mail.