The basic reason for separate disclosure of sales and purchase returns is to assist management to what extent their purchases and sales are subject to return so that it can identify the causes leading to these returns. The returns are usually due to reasons such as incorrect specification, incorrect quality, bad packaging resulting in damages to product.

You should remember that the accounting information must help the user to to take rational economic decisions.

these are shown separately on your P @ L statement

I’m not sure exactly what you’re asking. Are you asking why the purchases and sales returns and allowance accounts are shown as a separate grouping on an income statement? If that’s the question you’re asking, the reason is because those particular items are part of what is considered “Cost of Goods Sold”. Income (revenues) minus Cost of Goods Sold = Gross Profit. Gross Profit can have significance from a management standpoint.