Why is there a quote_address_id field in sales_flat_order_address table?

Question

Just stumbled upon the quote_address_id property of sales/order_address entity (sales_flat_order_address table) which in my case is always NULL.

Well I wonder .. what is is for? My only guess it is used at the stage when order is not yet finalised but I failed to find a proof in the code. Another guess may be backward compatibility.

The existence of this property/column is not critical of course. Just a question of curiosity.