Invoice Matching and billing

 I would like to know the business instances under which the invoices are over billed (matched more than the quantity ordered in PO), the system calculates price variance and generates proper entries. We had this option open for quite a while and there are user mistakes, now we are considering to disable manual release of QTY ORD holds. Before we conclude or decide anything we are contemplating all the scenarios where overbilling (matching more than the quantity ordered but taking the same value) will be required.
I just need the business reasons and examples where this option is useful (since oracle has allowed it)