I'm trying to understand the concept of functional harmony as it pertains to composing chord progressions, in order to compose my own progressions.
I've read a few articles and watched some videos, I think I get the basic idea that some chord classes lead to and from other chord classes (T, P, D). This image sums up what I understand so far quite succinctly. I got it from here.
Fundamentally though, I don't understand whether this concept is saying "Do this", or "Have you noticed how music tends to do this?"
Because if the latter, then the concept of functional harmony is a great ice breaker when trying to pick someone up at a bar, obviously, but not very useful for actually writing music.
If it's the former, and it's narrowing my list of "what should the next chord be" candidates while writing, that's great, but then I have to wonder why almost none of my favorite progressions fall in line with that graphic, as I understand it.
For instance, Creep, by Radiohead: G-B-C-Cm. The article tells me that I can move right as far as I like, so no problem going from the tonic to the III. Then no problem going from III to IV, we're still moving right. But then... we just go from IV to iv? Huh? I thought we were in G Major?
What about Jimi Hendrix's Little Wing? Em-G-Am-Em-Bm-Am-G-F(wtf?)-C-D. Why can we move from dominant to pre-dominant in the first place? That basically means we can move from any of our three chord types to any of the others, except for pre-dominant to tonic. But we do that in this progression anyway when we go from Am to Em. And then we go Am to G, and it doesn't say we can do that anywhere. Then we go to F? The worst possible chord we could pick, and it turns out to be the best, most sweetly harmonious sounding part of the progression? It's not a passing chord, he lays on the F chord for a bar, and plays F major pentatonic licks over it.
How does chord function explain any of that?
More importantly to me, how would thinking about chord function have helped me write those chord progressions?