About overview notes
In practice, overview notes develop very organically.
Imagine you are researching a topic, say: coercion in psychiatry.
Naturally you are reading many sources, lots of papers and such. For each paper you write down their respective key insights on individual notes. As one research paper can spawn not only one but several individual notes (“zettels”) and you don’t only process a single paper but many over the course of your research, I am sure you can imagine how fast you will accumulate a vast amount of individual notes.
Prevent notes from getting lost
Sure, the notes will link to one another via follow-up notes or cross references, so you can start with a random note and just follow the breadcrumb trail, but there will always be notes that are not within the cluster (or rather the referential map) you are currently navigating. So how would you reach those notes?
Prevent yourself from forgetting the connections
Another maybe even bigger problem is that even if the notes you write are all self-explanatory due to their atomic nature, you will still forget in which context you initially wrote them after a couple days or weeks.
And a note that you forgot the meaning of is basically worthless, especially once your Zettelkasten reaches sizes of well over 150+ individual notes (mind if your Zettelkasten grows even bigger over the years and you have several thousand zettels in there, which you eventually will end up with).
At last, the most immediate need:
Once you have written a dozen or so notes about your topic, you will want to use them to write, because that is what the Zettelkasten is all about and why you are using it in the first place.
You will want to use the notes to create an argumentative structure that will eventually become your own draft, so you need a place where you can see every note (everything!) at one glance, so that you can get the notes in the right order.
In contrast: If you had a paper based Zettelkasten, you would take all the relevant notes out of your vault, lay them out on the table in front of you, and shuffle them around to see how they relate to each other.
The overview note basically serves as your digital desk in front of you.
Here’s an example of an overview-/hub note from my Zettelkasten:
About Cross-referential links
The genius behind the Zettelkasten method is that, once your Zettelkasten has reached a certain critical mass, you will come across an idea that, while being in a certain context, will also apply to a completely different context. On the surface the two notes might not have much in common, but will share the same abstract principles that connect the two.
This way you can combine already existing ideas (that you wrote down on individual notes) and generate a new idea based of the connection or commonality the two share (which can in turn become a new research topic etc.).
At the moment I don’t have a good example in mind, but I hope you get what I mean 