How do cows help the environment?

They're herbivores, which means they eat grass like other wilder herbivores do. Eating grass encourages more grass to grow, and the grass and soil benefit from this, especially if cattle are grazed in a managed-intensive way that helps distribute manure and grazing more evenly than if they were allowed to graze a large pasture for one entire season. This type of grazing (the MIG grazing) really improves the soil (since no man-made fertilizer needs to be added), and it encourages grasses to grow more vigorously, thicker, and even encourages more grass/legume species within a sward. This type of grazing also mimics the way the buffalo grazed on the prairies long before the white man came along and nearly wiped them out, which is nature's way of encouraging more organic matter being added to the soil and carbon being sequestered into the soil.