My best answer:
Years ago, a farmer had 80 acres, or maybe a quarter section, and a handful of cows. His cows went from the barn, to the pasture, then in the fall he would wean or sell the calves. The cows/calves never left home, so to speak. About all he needed, was a brand, so that if a fence broke and something strayed, he had proof of who's cows they were.
Now, take myself, as an example. I am not in any way shape or form a large cattle rancher, but after calving I haul cattle out to 3 separate cattle pastures. Some ranchers take cows to maybe a dozen or more.
The ear tag makes it easy to pair up mama and baby, for when you are hauling them away from home. As the calves are born, I tag them with the same number that mama has, so that calf 123 goes with cow 123, calf 456 with cow 456, and so on.
This way, I can have a neighbor help, or hire just about anyone who can shoo a cow, to help sort out pairs to load out to pasture.
If you don't haul the baby with the right mama, no other cow willl take care of it.
Now, when 'farmer Brown' had his quarter section, with the back 80 fenced off, and only 20 cows to put there, he didn't have to worry about mixing up the pairs, because they were all in the same place. However, haul a mismatched pair off 5 miles, and you probabaly just killed a calf.
