In the air the airbrake can increase the rate of descent for a given airspeed, or assist in slowing the aeroplane while in level flight. On the ground the speedbrake (the same bits, but different purpose now) reduce the lift produced by the wing, thereby putting more weight in the wheels, thereby squashing the tyres, increasing the contact area of rubber on tarmac, thus increasing the effectiveness of the brakes. I have heard quoted that speedbrakes (or spoilers) increase the effectiveness of the brakes during the roll out by up to 30 per cent.