Why does the government need to round up wild horses?

the Bureau of Land Management is not corralling all wild horses, but they periodically do a roundup/ infertility shots for the mares. Some of them they let go and some of them they keep to auction off. The reason why they do this is to keep the population of wild horses small so that the land they roam, and land nearby does not get destroyed by over eating vegetation or by being trampled.