Document date field is used in free text invoices and it should be also related to the project invoices. Document date calculate right due date to the invoice and accounts receivable and is separate from real system invoice date.
This is important, when you have service industry invoices from subscriptions and you should invoice for example quarter based subscriptions before hand to get accrued revenue periodic run to work and customer agreement can be, that it should invoice at the end of the quarter period. Within document date we could follow up standard and invoice at beginning of the quarter with real present invoice date to get right accrued values for months, but calculate due date within document date and can use that date in the invoice layout.
There could be the parameter, which tell to the system, if invoice should use either invoice date or document date in layout and electronic invoice file (this part can be also customization - most important is that document date to be in use).
There is no possibility to do this document date field addition within extension now - so there is no work around to do this document date functionality.
Other reason for the document date is, that if customer request credit invoice to be within same invoice date than origin invoice. There might be situation that our period is close and within this document date, we could manage also this situation.