My guess is that it reduces drag while minimizing the negative impact a weaker rear wing can have on underbody efficiency.
Because the wing is aerodynamically coupled to the diffuser, whatever you do to one has an effect on the other. Moreover, all changes have upstream consequences, i.e. the diffuser and rear wing affect both the leading edge of the floor and the front wing.
Simply lowering the wing's AoA reduces downforce/drag across the board.
A Mercedes-style "spoon" wing featuring a lower AoA at each tip reduces drag by way of reduced downforce and minimized tip vortices, while the higher AoA of the wing's center section maintains a stronger connection to the diffuser's center section, an area where the crash structure probably makes the connection more challenging under any circumstances.
The Haas design seems like it will reduce drag across the board while maintaining a more consistent connection along the entire width of the diffuser. If so, it likely won't reduce drag as much as other solutions, but the rest of the car won't lose as much downforce.
