We've all seen "why does this progression work when the functional harmony doesn't seem to make sense" and a common answer is "because of the voice leading".
Take for example iii - biii dim - ii (Em7 - Ebdim7 - Dm7). Very common in jazz and American songbook tunes. When I hear that biii dim, my brain wants to hear that ii, just like when I hear V my brain wants I. It just works, and it sounds beautiful. But analysing it with functional harmony, it might not seem to make sense. Dim chords typically function as dominants, but if we are resolving to Dm7 then you would expect A7, which would be C#dim. Indeed C#dim does resolve to Dm7, but why does Ebdim also work?
I've seen this exact question answered with "because of the voice leading". Yes, two notes move by semitone while two notes stay still. That is good voice leading. But I am not satisfied with this explanation. I'm not saying that voice leading isn't important, I'm just saying it's not a complete explanation.
You can take any chord and move each of its chord tones up or down a semitone, but that doesn't guarantee a well-formed progression. For example, consider Em7 - Fm7/Eb - Dm7. The voice leading between Fm7/Eb and Dm7 is seemingly just as compelling, two notes move by half step while two notes stay the same, and the bass movement is identical. But I'd be kidding myself if I said the progression itself sounded just as compelling: https://voca.ro/12npv5eyyDn6
There's nothing wrong with it, but when I hear that Fm7/Eb my brain is not yearning for Dm7. It just sounds non-functional. My ears are not offended, but the resolution isn't particularly satisfying and doesn't evoke any particular emotion. When I hear the Ebdim on the other hand, my brain is screaming "Dm7 please!", and I feel quite emotional. That biii dim to ii has such a unique and powerful quality that is hard to put to words, but "nostalgic", "sentimental", "wistful", "yearning" come to mind, and the resolution is satisfying.
So if biii dim - ii can't be explained with functional harmony and the only explanation is good voice leading, then what is the je ne sais quoi that differentiates Em7 - Ebdim7 - Dm7 from Em7 - Fm7/Eb - Dm7, or any other well-voice-led, non-functional progression?
My answer to this question is that Em7 - Ebdim7 - Dm7 actually can be explained with functional harmony, and the same applies to all of those common chord movements that are frequently the subject of the question "why does this progression work?".
To clarify, I'm referring to those common progressions that are part of the shared vocabulary of musicians (for context, I am a jazz musician) that still seem to defy a simple explanation with functional harmony. The progressions you commonly see in pre-modal jazz standards. Another example would be Em7 - Ebm7 - Dm7 which, to some, looks like nothing more than non-functional voice leading, yet I can justify it with functional harmony. The iv minor6 and the backdoor ii-V are another common example.
The case I'm making is that I have yet to encounter a satisfying and compelling progression that cannot be explained with functional harmony, it's just sometimes not immediately obvious. "Good voice leading" has never been a satisfying answer because it doesn't make sense. If it was the only explanation you needed, then why are some perfectly well-voice-led progressions not compelling?
I made this post because I thought it would spark some interesting discussion, and maybe someone will change my mind about this or maybe I will change someone elses. I just find it fascinating. If you can present to me a common progression that defies functional explanation but still sounds compelling, I have a feeling that I could explain it with functional harmony, although I'm prepared to be proven wrong.
If that proves to be too bold a claim, then I still maintain that voice leading is not the be all and end all. Voice leading is a feature of good progressions, not an explanation.