Invoicing enables payment authorization for items received. You can add multiple receipts to an individual invoice and save approvals to an export file to feed to an accounts payable (AP) system. Locations receiving by invoice can combine this process with the Receiving process. If your receipt acts as your invoice (receive as invoice), then it is not necessary to use the Invoicing module.
Use receipt by invoice as much as possible because it does not require balancing multiple receipts to equal an invoice total. This might not be possible with all vendors. Consult your vendor about different options (COD is considered receive as invoice).