It’s running a very aggressive wheel size and offset on a car.
If you were to have an aggressive fitment on your Mustang, you could stuff some 18X13’s at all four corners with an offset that brings the wheel as far out to the edge of the fender as you feel comfortable with or what look you’re going for. Often a stretched tire and some negative camber is needed to clear the fender or that in combination with having body work done to allow clearance for the wheels. I’d say a zero finger gap or lower is minimal for a “proper” stance/fitted look. Wheels with low offsets usually are more exciting to look at, concave faces, bigger lips, or both.
It’s done for aesthetics. Pure visual appeal.
Look up www.stanceworks.com www.stancenation.com www.liberty-vip.com and so on.