I think this would clarify things for a lot of people here.
Historically, jazz is what happened when blues musicians went through an American high school education in classical theory, and so discovered the basics of harmony. Because the blues scale is the "color notes" from the diatonic scale, it changes key easily and requires just about no knowledge of harmony or melody. Consequently, there's about a dozen blues song archetypes and then you move on to another genre.
Most jazz songs were white popular music of the 1930s-1950s. This older style picked up some of the blues, but tended NOT to confine itself to that narrow harmonic framework, and so was good fodder for jamming. (I'm not saying music needs to be harmonic to be good, but I am saying it can't limit itself to color notes without RAPIDLY becoming FUCKING TEDIOUS. For example, Robert Johnson is great in part because of his use of "accidentals," but even that you can't listen to for more than a few hours without getting quite bored.)
Jazz, blues, rock, pop:
Verse \____ cyclic
Ancient music and metal, which are very similar, have no fixed song format; they often use verses and choruses, but as in the ancient Greek format, choruses usually occur after a number of exchanges of motifs composed of several riffs. In the best metal music, this is fully articulated.
Riff A \___ theme A
Riff D /
Riff B \___ theme B
Riff E /
Riff C \___ theme C
Riff F /
So a typical pattern might be:
And THEN comes the chorus.
If you look at tablature to Celtic Frost, Burzum, Slayer and Deicide, you can often see this variations on this pattern played out.
They are variations because in metal, as structured music, the pattern is defined by the content of the song. In contrast, in unstructured music, the pattern of the song remains the same and the repetition of a riff(s) achieves "mood." There is no journey in it. The starting point is the ending point, with a little poignancy in the bridge. It's more like propaganda than art.