What is the theory of paying the cost of fencing for cattle versus absorbing loss of cattle?

Paying for the costs of Fencing is cheaper than losing cattle that you have spent much time, money and effort in to keep them healthy and their bellies full and put good calves on the ground. Fencing keeps your cattle in and prevents losses, and takes less time to build than it takes to replace those that you have lost, and I don't mean by buying cattle, I mean by building up the herd again with replacement heifers.