Suggestion:
The benefit of the closing process is that completed Purchase orders appears with red status sign in your list of available Purchase orders when you access Purchase order pages in update and display mode.
If the Purchase order is closed, then we cannot use the same in receipt.
System will not allow to close Purchase order if it is having an unconfirmed receipt. User needs to close the Purchase order only before using the same in receipt. However we can cancel the receipt after deleting the recipt details.