By the legendary Leslie Lamport:
Architects draw detailed plans before a brick is laid or a nail is hammered. Programmers and software engineers don't. Can this be why houses seldom collapse and programs often crash?
Blueprints help architects ensure that what they are planning to build will work. "Working" means more than not collapsing; it means serving the required purpose. Architects and their clients use blueprints to understand what they are going to build before they start building it.
But few programmers write even a rough sketch of what their programs will do before they start coding.