The rear wheel dish is what Cannondale calls AI Offset. In addition to allowing increased clearance while keeping chainstays short, it also offers a likely stronger, more tension balanced wheel - that is, less of a tension difference between driveside spokes vs. non-driveside spokes.
Of course the downside is yes, as mentioned, a standard wheel needs to be re-dished the 6mm toward the non-driveside. And then it’s not swappable into a bike with standard symmetrical rear triangle. That said, any wheelbuilder/good shop will be able to do the re-dish in a matter of minutes (assuming the wheel’s in good shape to start).