You would think with bandcamp, soundcloud, etc that demos, early edits, and rehearsals would be plentiful. At least, I would. It doesn't cost anyone anything. Back then, the costs were minimal, but still, blank tapes, dubbing, preparing packages and letters, postage (often international). It required a lot more effort and time, yet they were relatively common. I guess you can reconcile that to "necessity is the mother of invention." You work with the situation at hand. I used to love getting that stuff. Not because it was exclusive or a symbol of trust and friendship, but to have insight into the creative process and some of the conversations they started. As for economics, because Discogs was previously mentioned, these type tapes were coveted as gifts. They were special. I know we don't like to think of this brutal nose culture as intimate, endearing, vulnerable, or generous in spirit, but that's how I perceived those packages. I wouldn't go as far as to say they were secret, but I know I never spoke of them with anyone. Would never sell them, copy them. To list something like that on Discogs? Discogs is also archival, not just a marketplace. The archivist in me would want to see them listed, but that's one of those deals where you ask permission. But asking permission is even rather bold. It's difficult to think in both the old way and the new way without dissonance.
I can think of two noise artists in particular who did this more than anyone else. And then there was Kawakami/Disclose who sent out so many "private" recordings that there's no way to keep track of them. If you were penpals with him, you knew you were getting demos in the mail. I don't know how he ever had time to walk away from his tape decks.