If you follow good SE practices, how do you develop software now??
  You may have formal or informal inspections of requirements, designs or code review. The quality assurance in this case is an after the event verification, i.e. you test the implementation to assure that it functions as its supposed to.
 
 
Many organizations have found that formal technical reviews or inspections find problems before they are part of a design are coded. Which leads to higher quality products and saves time because errors are found early during the development process.

A logical extension of this is to make a review or inspection-like process a part of the on-going development of a product. As usual, the goal of this method of developing software is to produce high quality software that meets the customer's requirements.