The quick answer to this question is “predictability”. A well-established breed will allow you to make educated guesses about things like milk production, feed efficiency, and even behavior. Nubian dairy goats, for instance, are known for being loud as well as good (but generally not phenomenal) producers of milk. Breeds of livestock selected for meat production build muscle and fat quickly. Leghorn chickens are flighty, spooky bastards but lay eggs like nobody’s business. In short, with purebred livestock you know pretty well what you’re getting.
Sanglant is the product of a cross between a Nubian doe and a Baylis line Spanish buck.
This leads, of course, to the problem of shrinking gene pools. Closed herd books are a double-edged blade (or, hah, a mixed blessing). As the average coefficient of inbreeding increases in a given breed, individuals become more and more prone to inbreeding depression. You’ll get animals who just don’t thrive, who have weak immune systems. At that point, careful outcrossing may be the only way to save the breed. Done well, it will preserve the breed’s essential characteristics while revitalizing the dying gene pool.
But I digress. Purebred livestock are important because they offer farmers predictability in their stock and when carefully stewarded preserve genetic resources handed down to us by our own ancestors. Pure breeds can offer us a glimpse of what our forebears thought was valuable and important to preserve, and should the major commercial breeds of livestock be endangered by disease or environment, other breeds may step in to save them with a genetic contribution or indeed replace them. The lovely variety of livestock breeds also allows farmers to select animals that will be economical to raise under whatever system the farmer has decided to use, from organic pasture-raised gourmet foods to backyard food sources to industrial production destined for supermarket shelves.