Invoice Matching and PO,Receipt.

Just had a question regarding invoice matching. Is it really necessary to  provide a PO information so that Invoice validation succeeds during validation process. Since we are giving the supplier and site information for a invoice,Cant the system go find the PO or receipts corresponding to that supplier information and then validate. From what I see unless we give explicit PO information (for online invoices), the system is putting a hold on the invoice,saying that it is not able to match.
In general, do we get invoice thru interface, with out the PO or receipt information?
Any thoughts??