Clearly, you can't release a payroll program incrementally. If it doesn't compute the paycheck correctly, it's of no use at all. What good would it be to have a payroll program that could get your pay right but couldn't compute your taxes or take out your health insurance costs?
Well, setting aside the obvious value of a payroll that didn't take out your taxes, let's think about this a bit.
If there's already a legacy payroll program in place, maybe you could replace the hours times rate part of it with the new one, allowing you to stop using one of the old COBOL programs and get some good out of the new improved calculations.
What about all those adjustment cards that have to be keypunched from forms filled out by the ...