But, it often isn't cheap to do, and in that case, it makes sense to prioritize. The high priority items for me are the things that I might want to share, the ideas I want to amplify for my contemporaries and future generations that might examine my life. Stuff like [1] [2] and [3] which has influenced my thinking fundamentally, that I hope to build upon so that others can build upon what I have built.
I'd argue that you do this intuitively: you're mentioning a letter from your family's past because it is a high priority item--it's relevant because it was the last written words of your great-grandmother's first husband.
But, there's a lot that isn't worth keeping. My first form of archiving as a teenager was keeping ticket stubs for movies and concerts--a decade later I was going through my pile and found that I didn't even remember most of them. The better movies, I remembered--and I had them on DVD. The better concerts, I remembered--and I also had journal entries and CDs to remember the experience and the music. It's not important to me where/when I saw Everything, Everywhere, All At Once in theaters, but I have it on DVD and I can't wait to show it to my niece when she's older. And sure, I saw Amigo the Devil live, but frankly, he's not an artist you need to see in concert--the greatest impact of Cocaine and Abel[4] on me was when I listened to it alone in my room. The ticket stubs simply don't matter to me.
[1] https://www.viridiandesign.org/notes/451-500/the_last_viridi...
[2] https://www.ted.com/talks/brene_brown_the_power_of_vulnerabi...
[3] https://digital.wpi.edu/pdfviewer/wm117p10z
[4] https://www.youtube.com/watch?v=ZzjtLm0G49E
EDIT: All the things linked above, I have backed up in one form or another. Notably, the Schutt paper isn't at its original URL.