Seiza is the name of a sitting posture in Japanese. People in Kung Fu classes sit seiza as well as regular people in daily life. This pose is also found in Indian Yoga and Buddhist meditation.
There is a reason for everything. Nobody does anything without a reason. Every cultural custom is done for some reason. You job, if you are a curious person seeking to improve themselves in this life, is to figure out what it is that is going on.
Here is a picture of a Japanese man doing the Seiza posture.
All of Japanese culture is designed to make the Japanese the best at whatever they attempt. This means that they do the Seiza posture for a reason that has to do with improving themselves. They don’t do it because it feels good or because it looks good. The posture will change or affect them in a very specific way. The posture has a definite concrete purpose unrelated to meditation or other mental pursuits.
The picture makes it pretty obvious