Dear Student,
F1 car is designed in such a way that air flows more rapidly over its lower surface than the upper. This creates an increase in pressure on the top surface compared to the bottom. The resulting pressure difference creates a downward pressure, which we call ‘downforce’. which generate more grip between tyres and road due to increase in friction
Second reason is that for streamlining is to reduce turbulence and air drag generated by airflow during the motion of car at speed because air drag disturbed car’s straight-line performance.