Ground effect aerodynamics of race cars is concerned with generating downforce, principally via low pressure on the surfaces nearest to the ground. Suspension motion leading to unsteady flow. It has been well documented that, at a particular incidence, running in proximity to the ground gives increased levels of downforce compared with the freestream case.
Why do you think f1 cars are on the floor? Time attack cars? Touring cars?
It's all to do with suction towards the floor and accelerating over the suction and ground pressure.
Put 2 cars next to each other the EXACT same, same aero set up, except one is high and one is low. Which is going to go round the corner faster?
That's why it matters.