Rare breeds represent history. Buttercup, a Highland Cow. Their long shaggy coats protect them from the harsh climate of Scotland. I'm sure you've heard of natural selection, where nature plays a role in what species of animal survives based on natural surroundings. For example, the Scottish Highland cow has more fur than the Watusi because it lives in a colder climate.
Domestication is a process where humans help decide the selection process.
Our cows today were bred to be smaller than their wild ancestors so they could be easily controlled and fenced. In order for the modern farm to survive, it must do what it takes to produce as much as possible with less and less pasture. In the United States and Canada, most of our milk comes from the Holstein cow. But, if you've ever seen the beauty of a Jersey or Guernsey cow, you know that these are worth preserving. Look how beautiful Neptune, our Horned Dorset sheep is, and just imagine a farm without such animals, simply because farming is easier with sheep that don't have horns.