Historians love archives. Archives are collections of unique materials available only at the archive and so when you visit one, you get your hands on materials that you cannot get anywhere else. Sure, some archives are moving to put their materials online. If you are one of the towering intellectual figures of the twentieth century or if you won the Nobel Prize single-handedly twice, your papers are online and accessible by anyone with an internet connection. But that process is expensive and most archival material is stored in boxes just waiting for researchers to come visit.
Those boxes can contain all kinds of things: diaries, notes, drafts of books, correspondence, pretty much whatever the person donated for preservation can be in there. And historians get to go through it all. Vern Jensen, a professor I knew at the University of Minnesota, called it, “reading other people’s mail and calling it ‘work.'”