Just a guess on my part, as far as the practice of Zen goes, koans seem to be very popular whereas zazen is kind of a labor: something physical that you feel you have to do which is not all that interesting at times.
Supposedly helpful to our understanding of Zen, I am not that sure that koans are all that beneficial or necessary. Before the Song dynasty (960–1279), koans as a specific Zen literary tool were not in use. There were no doubt personal interviews and sermons but nothing in the way of the koan as we now know it.
If the koans that we have are somewhat like snapshot of a face-to-face interview between a student, who may have walked many miles, and a Zen master, the interview can’t be all that valuable for latter day Zennists. A lot of personal context is missing such as the depth of the student’s learning at the time of the interview. Here Bankei (1622–1693) presents the problem:
"When worthy Zen masters of the past dealt with those who came to them, every word and every movement were appropriate to the moment. It was a matter of responding to their students and their questions face-to-face. They had no other purpose in mind. Now, there's no way for me to tell you whether that was necessary, or helpful, or not."
Koans, to be sure, are ‘head scratchers’ but are they all that helpful or necessary to Zen? Zen in its proper context is about seeing our true nature but how does this relate to koans? Without further instruction by a competent Zen master on how koans work, to help the student attain their true nature, we are more or less working in the dark.
The attraction that koans have can even pose a danger for the average student. In other words, koans can be taken the wrong way. Granted, if we awaken to our true nature it doesn’t take us long to see the design of koans and how they skillfully demand of us kensho in order to understand them.
This is so with the koan, Nansen Cuts the Cat in Two. Nansen stopped the fight over a cat by the monks in the eastern and western halls by seizing the cat then demanding the monks say the ‘right word’ that would save the cat from being killed. This is really a demand for awakening to our true nature. If a single monk actually saw their true nature they would know, instantly, what to do showing the expression of Mind essence. None could. Later than evening when Joshu returned Nansen told him about the incident. Joshu simply removed his sandals, put them on his head and walked out. In effect, Joshu demonstrated the ‘right word’. What is unfortunate about koans like this, we might wrongly assume that Joshu’s response was absurd—and this is the right word. But it wasn’t absurd anymore than when the Buddha simply held up a flower and blinked transmitting the Dharma to Mahākāśyapa who just smiled.
If I were to put this same koan into a more transparent form which the readers are used to, it might be easier to see what is really going on. Nansen is really demanding of the monks, demonstrate the animative principle and you will save the cat. For Joshu it was easy to demonstrate this mysterious principle. He simply put his sandals on his head and walked out—and let me add—the Buddha simply held up a flower and blinked transmitting the Dharma to Mahākāśyapa who just smiled. (Emphasis is mine.) What makes this even more true is to realize or awaken to this true nature, that is, the animative principle which is luminous.