I have noticed with my own compositions that I can take something like Bb major and use it to prepare for a C minor entrance, even if C minor is in fact the tonic. Also I have noticed that I can use G major as a preparation for Bb major even though relative to Bb major, G major has the the third augmented and is thus not diatonic. That alone increases tension more so than Gm so Bb is even more desirable of an ending.
But in C minor, Bb major is completely diatonic. And that is for the natural minor(which I use all the time, I never use melodic and very rarely use harmonic).
So for example this chord progression, which I am thinking of using to base my sonata development off of:
C Am F G Em Fm Bb Cm
Sounds perfectly fine, here is the Roman Numeral analysis:
I vi IV V vi/V iv IV/IV i
That IV/IV works perfectly well as a secondary dominant. You could also consider that to be VII or the subtonic. I think I know why the subtonic works so well. It has to do with that whole step. Same reason ii V works well in a major key. It is a whole step away from the tonic and is acting as a secondary dominant. And since secondary dominants can act as primary dominants, Bb to Cm with Cm being the tonic is like an inverse retrograde ii V if you will.
Inverse because of 2 reasons.
1) Instead of going to the dominant, you go to the tonic
2) The secondary dominant in the ii V is minor and in the VII i is major
Retrograde because in a sense, you are going backwards. The ii V increases tension and the VII i releases it. So if you base a piece off of ii V progressions, every additional ii V adds tension and you reach a climax where you could snap back to the tonic. VII i though is perpetual. There is no way you can get more relaxed sounding than ending on the tonic.
But why is it that in major, vii has no clear harmonic function as just a triad and requires a 7th chord context whereas in natural minor, even with just the triad alone, there is clear dominant function and no 7th chord is needed? Is it because in major, vii is a diminished triad and diminished triads have ambiguous resolutions whereas in natural minor, VII is a major triad and a major triad has a clear destination?