I know the least about them as anyone, so I've provided a link to their genesis.
The original BMW design may have been predicated upon the entire front of the tires being inboard a bit, and shielded by the fascia, in the 'lee' of the wheel arch, inner fender well, with zero air actually attacking the tire face at all.
And I suspect that BMW was looking for a specific air velocity in the 'curtain', with respect to road velocity / air velocity.
https://www.press.bmwgroup.com/globa...nology-11-2012