That medicinal herb was called the Meridians Cleansing Grass. This herb had a special medicinal effect on the process of meridian cleansing. It was not only effective on humans, but it was also very effective on monsters. If a young monster were to consume it, it could also perform the process of meridian cleansing on the youngling.
Therefore, there were often powerful monsters wandering around the vicinity of the Meridians Cleansing Grass. Although they did not have any need for the grass, their descendants and the younglings needed it.
Although they did not have a systematic understanding of the Meridians Cleansing Grass as humans did, the knowledge passed down from generation to generation allowed them to understand the benefits of the grass.