A lot of entrepreneurs have an idea and then start looking for a developer, build something and launch it only to discover that users are not connecting with the product. Doh! It would be far more efficient to do a product discovery phase before consuming engineering cycles. Simply put, product discovery makes every product manager much more efficient.
See also my interview with BostInnovationhttp://bostinno.com/2011/03/26/3-questions-with-iphone-guru-giuseppe-taibi-on-mobile-project-management/