In a minor II-V-I progression, we usually have a IIm7(b5) chord followed by a V7(b9,b13) chord, resolving into Im7. I always thought the diminished 5th in the II chord (also the minor 9th in the V7 chord, same note) was like a "hint" that a minor chord was coming, since it corresponds to the minor 6th of the I chord. That would be F in A minor/C major.
If you analyze the progression from a relative major standpoint:
VIIm7(b5) _ V7/VI(b9,b13) -> VIm7
In a major harmony, the VIm7 is related to the Aeolian mode and should have a minor 6th, right?
However, it came to my attention that some standards, like Autumn Leaves and Caravan (no II here, but a long time spent on V7b9), have this chord written and played as Im6 sometimes, with a major 6th instead. Is there a particular reason for this?
Edit: Is this a chord substitution? Some substitutions are based in harmonic concepts, like the tritone substitution. Does this major 6th thing have such an underlying concept?