The ones they claim to have lost were the ones that were between her and other government agencies. Even if they were being stored on local hard drives (not likely unless it was just a cached copy) at worst they’d only lose the ones she’d received. The ones she sent would be on the recipient’s computer.
If only things were that simple. The book, The Whistleblower by Marinka Peschmann, 2011, has a good narrative at all the tricks invented by the Clintons, and perfected by Hussein, to evade the transparency they both shouted from the rooftops, pounding their chests ( except, perhaps, Hillary).
Using private computers on clearly national interest matters, to evade the recordkeeping required by law, is a good example.