Proforma/ Commercial invoice for EXCISE INVOICE

Dear friends,

Excise Invoice is created with ref to a proforma Invoice as per the standard SAP. But SAP also allows to create excise invoice with referrnce to the Delivery doc and also Commercial Invoice.

Can some one help me to understand the pros of cons for referring proforma for Excise invoice creation and why not Commercial invoice so that we can avoid one step in between?

Thanks

Isaac