Why is it important to take a physical inventory periodically when using a perpetual inventory system?

In Financial Accounting I, you learned that the cost concept of accounting requires accountants to record all items purchased at their cost. In Chapter 7 of your Cengage Accounting eText, you learned that inventory may be written down to its current replacement cost or its net realizable value if these amounts are lower than the original cost.

Why do you think the accounting profession has decided to violate the cost concept and reduce the value of inventory in these circumstances?