An external structure is needed to surpass the limitations of brain
We need a second brain to complement our natural brain. Our memories are finite and our capacity to store information is limiting. The human brain is wonderful when it is doing what is supposed to do. But when you force it to be used as an information process storage tank it fails. Our brains are good for two things making decisions and pattern matching (or linking)
External systems help our brain to offload the stuff that is being thrown into a reliable place. Systems such GTD uses it to offload a stream of information so that you can make a decision later when you are free to process the information.
GTD functions as an extended memory system to keep track of the things that are important to us. GTD helps us make better decisions without being overwhelmed by the storage of this information.
But as a second brain Zettelkasten goes beyond that. It is not just a place to offload information. It is designed in such a way that the offload occurs contextually. Zettelkasten not only helps you with decision making but also with pattern matching. It becomes a place for you to play with ideas so that you can connect the ones you feel will fit.
It becomes a conversation partner.
The similarity between GTD and Zettelkasten lies in the fact they are both external systems and offloading stuff into them requires a considerable amount of habit development. GTD takes lesser overhead when it comes to decision making while Zettelkasten is more taxing that way as you are decision making right from the moment you are reading something. This initial taxation will pay its dividends in the long run as you can generate new novel connections and thus becoming perfect examples of how and the power of s. It becomes ingrained in a way.