My partner works in historical archiving for science and medicine. Museum work, basically. He's told me so much of the archives are donated collections of notes, letters, journals, and so on from important doctors, researchers, scientists, etc. Donated by the subject themselves in their later years or by their families.

He's told me there is a growing issue with those people starting to donate entirely digital collections, but even worse than that, are all the documents that are not being stored on a physical hard drive, but on web services and clouds. By the time these people are willing to start donating their things, so much of it has just been deleted forever without them realizing it. Or worse, they die, and their families no longer have access.

Working in IT, I told him about Microsoft's growing push to eliminate Outlook and PST files, make it all web based email, and he wasn't surprised, but he was still bummed to hear it. Apparently a not insignificant amount of those donations are locally stored emails.