When grass is at its peak production and quality, cows are at their peak requirements. Less manure hauling . Cows leave it in the same field where they eat grass. This leaves only a small area around the barn to be scraped. And spreading the manure on the entire farm reduces the amount of commercial fertilizer needed Reduced chance of mastitis and lower cell counts.
Cows stay cleaner on pasture as long as they are rotated properly.
Having cows dry in the winter helps eliminate frozen teats, scabs, etc. Our cows are outside year around with cell counts running 150 -170,000. Less veterinary expense. Cows on pasture are healthier and are in better physical condition. Less fuel and repairs.