Whenever there is a news story srrounding archive.org or any other kind of preservation society a discussion ensues about the supposed value of preserving e.g. every lesser piece of news or art, regardless of our current valuation of it.
I see then the value of having such public information preserved and think to myself that public archives do a well enough job to pass this heritage on. Though then I remember that personal letters or diaries are used as sources too, quite personal information.
So what kind of things should I preserve for future generations that is not public information in the interest of historians?
I realise the question in the body is different from the question in the title but they are related and the body gives some context.
I have no idea what's best to preserve, neither do you, nor will anyone else posting here. That's because we have no idea what types of questions historians will be asking about the past in, say, 40 years just like the types of questions historians are asking now are different from those 40 years ago.
Let's even set that aside for a moment. The very act of you trying to save what you think will be of interest in the future is an imposition of bias onto your own preserved collection of records. It is shaped by your personal experiences, ideologies, circumstances, and values. It bends to your decisions about what you think is and will be useful. What you leave behind will not be some objective snapshot of your past as much as it will be a manufactured vision of yourself combined with your expectations of the future. Not that this won't be of any use to a historian, but leaving behind something contrived isn't any more useful than what you will incidentally leave behind anyway. The best and easiest way to go about this is to simply let the historians worry about the history.
But, it may interest you to know that historians do not themselves make the majority of the decisions regarding what records get preserved and how. That decision is instead made almost entirely by archivists who aren't any better at guessing what historians are going to care about than you are. In fact, they literally and consciously gave up trying some time around the start of the 1970s, give or take, when the archival and historical professions diverged sharply from each other for the first time.
On top of that, do you realize that since before even then, the "public information" that you assume is being fully preserved hasn't been archived in its entirety? Not even a fraction. There's just too much stuff created since before there were even computers (and not just think about how much more is being created since the invention of computers!). Most of what archives have been keeping all this time (and once again, fractions of it rather than the whole) revolves around records detailing institutions and organizations. Most of the personal papers and records detained tend to be affiliated in some way with those institutions (like a person who worked for it). Very few archives, if any, keep just any records they can find from any person they come across. There's too much stuff and they don't have the space, time, money, or personnel to process it all. It may very well be that the records you try and retain may never end up in an archive because no one sees enough value in it to process it (please understand this is not a reflection on you at all, nor your hypothetical historical value. I'm just trying to illustrate how little of the total historical record is actually retained).
Archives and the processes they employ are a surprisingly fascinating topic. A lot of people (even historians) assume that everything is being recorded somewhere but that's actually a fantasy. An absolutely fantastic book on the subject is Processing the Past by Blouin and Rosenberg.