The public learned on March 10, 2015 that Hillary Clinton had more than 60,000 emails on her private email system, and that she had turned over "about half" of them to the State Department and destroyed the rest, which she said were "personal" and "not in any way related" to her work as Secretary of State.
The public learned later the lengths to which Clinton went to make sure the "personal" emails were completely and permanently deleted. Her team used a commercial-strength program called BleachBit to erase all traces of the emails, and they used hammers to physically destroy mobile devices that might have had the emails on them. The person who did the actual deleting later cited legal privileges and the Fifth Amendment to avoid talking to the FBI and Congress.
Clinton's lawyer, David Kendall, told Rep. Trey Gowdy, chairman of the House Benghazi Committee, that investigators could forget about finding any of those emails, whether on a device or a server or anywhere. Sorry, Trey, he said; they're all gone.
It was, as the New York Times' Mark Landler said in August 2016, the "original sin" of the Clinton email affair — that Clinton herself, and no independent body, unilaterally decided which emails she would hand over to the State Department and which she would delete.
Still, there were people who did not believe that Clinton's deleted emails, all 30,000-plus of them, were truly gone. What is ever truly gone on the Internet? And what if Clinton were not telling the truth? What if she deleted emails covering more than just personal matters? In that event, recovering the emails would have rocked the 2016 presidential campaign.
So, if there was an enormous trove of information potentially harmful to a presidential candidate just sitting out there — what opposing campaign wouldn't want to find it?