Moving the cows, in Management Intensive Grazing (MIG), mimics the movement of large herds of bison moving across grasslands. Because a domesticated herd is not so big and our cattle feel safe, they do not stay very close together and completely graze one spot before moving on as a wild herd would.
In a large paddock, a herd of cattle spread out. Each cow will pick the best forages and eat them down to the ground. This allows less desirable forage species and undesired species to thrive as the cattle avoid eating them.
Additionally, when cattle are allowed to continuously graze a large paddock, they often find a tree or other favored spot where they lounge. They drop huge amounts of manure in their favored lounging spot. They also compact the soil from their continuous presence and the constant trampling kills all the forages in the spot.