Thinking About Prototyping - Paper Prototyping: The Fast and Easy Way to Define and Refine User Interfaces - page 52

Summary
Although paper prototyping isn't a perfect technique, it's useful in a wide variety of situations. Its main
benefits are that it:
Is fast and inexpensive
Identifies problems before they're coded
Elicits more and better (i.e., less nitpicky) feedback from users
Helps developers think creatively
Gets users and other stakeholders involved early in the process
Fosters teamwork and communication
Avoids opinion wars
Chapters 12 and 13 go into more detail about the kinds of things that paper prototyping is and isn't
good for, as well as how people react to it.