Campbell's book is a must-read for any writers or filmmakers, because he does such a good job discussing something that is intuitive, but also difficult to understand. Plus, he did the added benefit of provide academic warrants for his claims by studying world mythology to show that it truly is universal.
But it's not just a helpful tool. It's also a call to action. Campbell's book shows the reader, "Hey! Look! All the heroes are just regular people who choose to leave their comfort zone and do something difficult."
It's a call to be a hero. There are also important religious questions raised by the work. It might be meaningful that each human life follows a rhythm, an ebb and flow of growth and pain, of adventure and home. This book, properly understood, is an invitation to ponder the deeper rhythms of life and probe them for meaning.
Also, there is a call to art. So many people want to become writers and artists, but they don't see the bigger picture, so they never start. With The Hero With a Thousand Faces, anyone should be able to write a compelling story, because Campbell has shown that the goal of literature (at least in this philosophical school) is not to invent something new, but to remind the audience of something old, ancient and powerful.