Why do vegetables mean dredging?

Vegetables have the meaning of dredging and come from food materia medica. Lu He's Materia Medica of Food pointed out: "Vegetables have the meaning of dredging. If you eat them, your stomach should be smooth and not stagnant." Then how does the meaning of vegetables come from "eating herbal medicine"? There are many works on Edible Materia Medica, among which Xueji has written two volumes. Lu He wrote four volumes and Wang Ying wrote seven volumes. Qian revised Dong Yuan Shi Ben Cao. Edible Materia Medica and Compendium of Materia Medica are regarded as two bright pearls in the treasure house of traditional medical culture in China, and they are classic ancient books of traditional medicine in China.