A persistent motorsports issue (and not only with stock cars) is the aerodynamic passing problem. You can’t pass without grip. Grip is a direct result of downforce. Downforce comes from two places: the weight of the car (mechanical grip) and the billions and billions of air molecules hitting the car (a.k.a aerogrip).
Running on underinflated tires can be dangerous. Underinflated tires they create more friction and more heat, which leads to not only bad handling, but also can produce structural problems.