I have 763M wheels with PS Cup2s that came on my M3 ZCP as a factory option. CS and 763M set up is the same tire sizes as GTS, but narrower wheels by 0.5 inch.
I think the CS setup works amazing. I feel that smaller front wheels allow for improved ride and feel, while the large/wide wheels in the rear help reduce oversteer and increase grip (maybe also improve handling/rear feel). I feel the CS/763M has the ideal set up but many may not like 19” front and 20” back. I love it though and wheels look great.
And to finally answer your question... I think they are cambering the tire sidewalls for cornering to improve turn in response. I remember the old E46 M3 ZCP had really aggressive sidewall setup with PS2 (stock) and I felt it was for improved turn in at the price of harshness. It might help to slightly reduce tire walls from moving during cornering (just a guess, not an engineer anymore).
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