One of Luhmann's most complete statements about his Zettelkasten system was an article in 1981, titled "Kommunication mit Zettelkästen". In it, Luhmann said the slipbox was a "communication partner" which had many of the attributes of a separate person. The foundation for this idea seems to be that it's useful (at least for some researchers and authors) to discuss a project with a knowledgeable peer who may have slightly different interests and may ask questions that open lines of inquiry that may not have been on the researcher's mind or completely in focus. The slipbox is NOT a complete replacement for a thoughtful interlocutor who can inject information you don't know or into the conversation, of course. But it CAN connect the researcher with her or his "past selves", the information those selves were interacting with, the trains of thought they were pursuing, etc. And by doing this, the slipbox can surprise us -- or at least Luhmann has testified to that effect.
I think this is an interesting idea, if you pursue it. That our prior selves are different people. We have an illusion of Continuous Personal Identity, but psychology and philosophy (https://iep.utm.edu/person-i/) have some things to say about that, which I'll leave for another time. My point at the moment is, we can learn things from the other people who were us just as we can learn things from actual other people. And to the extent that it's a mental map drawn by those previous selves, the slipbox can put us in touch with those others. Luhmann says one of the "most basic presuppositions of communication is that the partners can mutually surprise each other." Of course, this is a one-way process when dealing with the card-file, but the point is probably still relevant. It does seem to be the major value-add of the system, that it leads to unanticipated insights.
I think the biggest takeaway from this article is Luhmann's statement in the third paragraph that "It is impossible to think without writing." He seems to realize this is a reach, so he immediately qualifies the statement and says, "at least, it is impossible in any sophisticated or anschlußfähig fashion." The German word can be translated as "connectable", but many readers today say "networked", since that is what the bidirectional links of the slipbox basically lead to. (I turned this into a permanent note)
Luhmann's definition of information includes that it "originates only in systems which possess a comparative schema." Accepting that for the sake of argument, it seems like different people (or different versions of ourselves) would make different comparisons based on their varying interests. Asking different questions, following different trails of connectivity, might be subsets of this "comparison" idea that are easier to focus on, in this context. I think it's pretty clear how that leads to Luhmann rejecting what he calls "systematic ordering in accordance with topics and sub-topics" in favor of the numbering scheme he decribes as "a firm fixed place". This probably includes avoiding the date and time format, since there's probably some information present in when we think of things and in what order. This could be data captured within the card, but probably shouldn't be the top-line numbering scheme.
The key benefit of having these discrete cards in the box, relating to basic ideas, is that I'm free to link them in any way that makes sense to me. Luhmann calls this both "arbitrary internal branching" and "internal growth". He talks about flexible Verweisungsmöglichkeiten (possibilities of referral or linking), and urges the reader to be sure to "capture the connections radially" and record back-links. This is how the system gains its power to surprise us, I think: because these are the links we tend to forget between these pieces of information, that we'll benefit from rediscovering in other contexts. Context seems to be key, and seems to be one of the things that we lose sight of if we don't leave these traces in the system.
In the middle of the article, Luhmann once again says the slipbox "gets its own life, independent of its author." How far one wants to follow this idea, I suppose, depends on the extent one thinks the (conscious?) mind is a network of collected memories. If this is the case, then it might be correct to think of the slipbox as a "second brain" that we can use to "think differently". The box itself, of course, has no will. So as much information as it contains and as many connections, it won't MAKE those connections for us. Maybe I'll feel differently if I have an experience like Luhmann seems to have had, where the slipbox "tells" him something so unexpected that it seems like someone else talking.
What I DO buy, and what seems extremely valuable to me, is the idea that I'll be able to see "complexes of ideas" and hubs of network traffic that will be clues to me, allowing me to observe my own interests as they develop and change over time. Luhmann talks about "incidental ideas" that gradually expand and are "continually enriched" with new connections and new adjacent info and ideas, until they "dominate the system." I've been focused lately on looking at (and counting) the connections, because I think that will tell me a lot. I think the quality of the connected information is probably very relevant too. But it's certainly true, I think, that a LACK of connections is a sign of weakness. Along these lines, in the long run it's probably better to avoid connecting notes via tags. I imagine this would tend to short-circuit the process of truly seeing how these ideas and data relate to each other. It's another instance of top-down imposition of order that probably hampers the organic emergence of self-ordering.
Luhmann seems to believe one of the slipbox's unique values is this ability to "look for formulations of problems that relate heterogeneous things with each other." The slipbox's "memory" utilizes this network (and the network effect), returning much more useful information than a simple return to my library shelves to open some books, read the highlighted and noted sections, and try to reproduce the state of mind and understanding I had when I first made them, as well as recalling the questions that were driving me at the time. This gives me immediate access to the whole elaborate sort-of conspiracy-theorist map of this information I may have made at the time, complete with the little yarn-connections.
I actually wish I had some of this available to me regarding prior research. I did a LOT of work on Victorian London when I was researching a possible biography or historical novel about Charles Bradlaugh. I still technically HAVE all that information. But I'd have to reproduce the STRUCTURE of my understanding of London by rereading all those sources. It would be much better if I had a model of that knowledge structure that I could return to and walk around in, to regain my bearings. I'm going to try really hard to build this structure as I work on my current research project, so I don't lose my work again.
Luhmann concludes by talking about the role of accidents or serendipity in scholarship and in life. The slipbox, he says, "provides combinatorial possibilities which were never planned," and which can act in a more or less evolutionary way, "producing accidents with sufficiently enhanced probabilities for selection." As I was reading this I was reminded of Taleb's idea of antifragility being about benefiting from both surprises and from time. Luhmann concludes by saying it is not "methodologically meaningful" to isolate origin and value or (apparent) order and randomness from each other. What we should be concentrating on, I think he implies by returning to the claim that this is an "empirical investigation", is the result.