I know that this is a very simplistic drawing without proper labels for the forces and it ignores the car squatting in corners and some other forces...but
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Let's assume the car is cornering (to the right) with a very stiff wing, the downforce acts, well, downwards, the load on the tyres isn't equal since it's transferred to the outside tyre (left rear) so you're not utilizing the inside tyre as much as you could.
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With a wing which pivots you'd partially counter the car's squatting and the downforce would slightly counteract the uneven load distribution meaning more grip.
Whether that wing is tilting enough to have any positive effect in fast corners which the uncontrolled wobbling wouldn't nullify in slow corners is another thing to consider