The standard path of the mythological adventure of the hero is a magnification of the formula represented in the rites of passage: separation--initiation--return . . . .
The last couple of months have been telling me to read Campbell's work: I read Storytelling for Lawyers, watched episodes IV--VI of Star Wars, and then after doing some internet research of Dan Harmon (while trying to guess whether there would be a Community movie), I found an entire series of articles by Harmon about story telling. See here; here; here; here; here; and here. All of these are heavily influenced by Campbell's theories. (One might even say, these were my call to adventure! Get it? See what I did there? Get it?).
In college I was introduced to Campbell's theory during a class on German myths. It was interesting and straightforward, and obviously applied to a substantial number of stories. So, since then, I did not think reading the book worth my time (I assumed the cursory treatment was good enough for my purposes). I was wrong.
Campbell's theory is much more than a formula that applies to many myths. It is a theory of human nature, and explains how myths illuminate our understanding of that nature. That is, the monomyth parallels the human experience, the human journey; and so understanding the monomyth helps us understand ourselves.
This helps to explain why the monomyth has such universal application: by reflecting a pattern that applies to myths from all over the world and spread over different time periods, the monomyth reflects something fundamental about how we understand the human experience. We begin in a status quo, we leave the status quo, we return to a changed status quo. This parallels non-existence, existence, and then non-existence (i.e., pre-life, life, and death). It also parallels the journey of trying to improve or change anything.
Although it is not a goal for Campbell, the book has a lot to offer to anyone who wants to understand story-telling. This, I think, is why his work was so important to George Lucas in creating Star Wars and Dan Harmon: the formula that Campbell explains also explains what a story is. And, although you can follow it step by step (see Star Wars), you can also just follow a basic version of the formula to create television (see Community).
Incidentally, I've found the work to illuminate even my legal writing because it's helped me conceptualize effective story telling as compared to ineffective story telling.
One last reflection: there is an eery way in which a lot of major anime I can think of follows Campbell's monomyth, almost to every single step. I'm thinking specifically of Dragon Ball Z, Naruto, Evangelion, and, a show I've been watching on Netflix, Magi. Of particular note, these shows all include some sort of confrontation/interaction with the mother/father figures. I don't know what to make of this, other than it's interesting.