Parental benefits can begin after the child has been born, or in the case of adoptions, when the child has been placed for the purpose of adoption.
The concern is that if benefits were advanced up to 15 weeks in advance of the expected due date, and if the child were to arrive late, another week or two subsequently—if you start at week minus 15, now you've reached week zero—maternity benefits would end and your leave would actually also end. You would be expected back at work because you would not yet be eligible for parental benefits or parental leave.