On Thursday I asked you to review the fadeout of the Beatles' "Hey Jude," which famously features the [seven]♭ chord. The full chord phrase is:
[one] - [seven]♭ - [four] - [one] -
That great singalong chant melody in "Hey Jude" does not include the subtonic 7♭note, so it wasn't the melody that drove the Beatles to use [seven]♭ instead of [five]. And the rest of the song uses [five] and the regular ionian chords. They pointedly switched to mixolydian chords just for the fadeout of "Hey Jude."
Try singing the "Hey Jude" chant with these alternate chords:
[one] - [five] - [four] - [one] -
You'll notice that those alternate chords work just as well with the melody, but they bring a very different feel to the phrase.
It's hard to remember how groundbreaking "Hey Jude" was in a time when rock hits were trimmed to a length of 3:15 or shorter to meet the demands of radio. Since then, lengthy repeating fadeouts have become a staple of rock music, but at the time it took the biggest stars in music to break the mold and give us a 6-minute-long song with a couple minutes of repetitive ecstatic chanting at the end.
I think the Beatles' choice of [seven]♭ instead of [five] for the fade might make the mood a little more mellow, to help you surrender to the trancey groove instead of becoming impatient with the repetition. It's just my theory, and we'll never if "Hey Jude" might have been just as successful with different chords. But sometimes it's just this kind of subtle thing that makes the different between a good song and a great song.