Katsuo no namari-bushi has been deeply rooted in Japanese food culture for centuries. It already existed during the Muromachi period and was valued as a preserved food. By boiling fresh bonito and smoking it, the umami is concentrated, making it possible to store for long periods. In the Edo period, it became popular on the tables of common people, and even today, it is cherished as a health food.
The store manager has also been a long-time user. It's been about 10 years since I started eating it regularly. While other factors may also be at play, I've developed a "less prone to gaining weight" body type. Just as the fat content in bonito is thoroughly removed, this ingredient surely helps in reducing my excess fat.