We are all one kind of thing.
Campbell's work, although properly academic, is also religious in nature. As a religious person, Campbell found meaning in the universal story that underlies each person's sense of self. The monomyth is an account of how challenges and tragedies spur us on to become better, stronger versions of ourselves, and through art, we have the ability to remind each other of that essential quest.
The human life has a story arch.
Campbell believes that the unified myth, which has always existed and exists in every culture, is evidence that each human life has a calling on it for them to be the hero of their own story, to go through the steps to become reincarnated into better versions of their selves.
Go on the adventure.
Part of the monomyth is the home town of the hero. Many times, there are many people who could also go on the quest, but who don't. That means that the monomyth is a challenge to reject the status quo, to "follow your heart" and go do the things you are scare you aren't strong enough to do.
The adventure is the means of growth.
There is a scene in the Hobbit where Bilbo finds himself in a dark cave surrounded by baddies. But, along the way, he has gained two important tools: a ring to help him not be seen by evil beasts, and a glowing sword to help him see where the enemies are, even in the dark.
The meaning is that without going on the adventure of life, we won't be properly equipped to deal with the seemingly impossible, terrifying challenges that may lay in any of our futures.
