inventory written off or written down

Hi,
my question is why the cost of sales should include inventories written off or written down?
As far as I understand the meaning of COGS is cost of goods SOLD. But why should we include the amounts (written off or written down) into it that is not related to Sales?

Here are the text from ACCA F3 book:

The costs of inventory written off or written down should not usually cause any problems when calculating the gross profit of a business, because the cost of goods sold will include the cost of inventories written off or written down, as the following example shows.