To make cool tire designs on dirt
But seriously, to add more friction between the tyre and the ground for
grip so the vehicle won't easily slip while speeding up/down/stopping or steering sideways. On a plane and smooth road, these tire grooves also act as sort of air lock at high speeds adding more to the vehicle's grip to the road/ground due to pressure of air hitting the car which act as the downward push, pressing the vehicle to the ground and hence, the tires...
That's why ~200km/k speed is enough for a plan to take off the ground but a car at 200km/h or over doesn't fly. Basically its shape dependent.
and that's exactly why people change tires after sometime, especially when the grooves on the tire are worn out or flattened.
Edited by Faisal, 12 April 2012 - 03:39 PM.