Self-invoicing is to be done when purchased from an unregistered supplier, and such purchase of goods or services falls under reverse charge. This is because your supplier cannot issue a GST-compliant invoice to you, and thus you become liable to pay taxes on their behalf. Hence, self-invoicing, in this case, becomes necessary. Also, section 31(3)(g) states that a recipient who is liable to pay tax under section 9(3) or 9(4) shall issue a payment voucher at the time of making payment to the supplier.