It isn't just the classified stuff, ordinary people woulda got thrown in prison for that, but also all the information on who she sent and received emails from even if they didn't contain any classified information. It's like giving the Russians and every other swingin' dick a list of hundreds or even thousands of names to research and hack into. From them you get other names and IP addresses and the first thing you know the bad guys knew an awful lot about who to target if they want to mess with our 2016 election. And our state dept and no doubt other agencies. Which despite protestations to the contrary they certainly did. People focus on the classified data and yes, that was super important but it's also just one way among many that Hillary exposed our political system to attack.