Two reasons - at the time the American Quarter Horse Association was weeding out horses that had inherited a serious hereditary problem. Horses that were not tested were not allowed to be bred (not sure if this is still true, I am out of the breeding business).
The second reason - two of my mares foaled the same night. I saw and photographed the mares with their foals before dawn just after birth when they were still wet. By dawn, the foals had swapped mamas. My vet didn't believe me and I knew any further pictures I would have would have the foals with their adoptive dams.
So I paid for genetic testing - AQHA had it for parentage verification as well as for testing for the genetic disease. Of course, I was right - the two fillies at some point imprinted on the mares who were not their mamas.
Later I found out that the reason AQHA was providing parentage verification was that when they began testing for the genetic disease, they found many cases of foals not being sired by the stallion they were supposed to be or that mares in the same herd had done what mine did. The first set of cases turned out to be deliberate frauds - owners of famous stallions over booked and had the mares bred by lesser stallions and still collected the high fees for the famous one. It really shook up the organization for a while.
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