What's the use of requirements engineering?

There are many ideas about how to do requirements engineering, and often they conflict with each other. Such conflicts can best be resolved by asking of anything one proposes to do, "What is the use of doing that?" The question demands a thorough understanding of the principles behind different methods, and the answers may surprise those who equate pragmatism with informality. I discuss how applying this rule helps in choosing requirements engineering methods and in dealing with the difficulties that arise in applying these methods.